General guidelines
For the purposes of this article, I'm doing to use "disabled" to mean "finds it difficult or impossible to do things that someone might ordinarily expect a sex partner to be able to do". This is a very context-specific definition. I'm also going to use it as a blanket term for short-term, intermittent, and permanent disabilities. For more information on the language used around disabilities and the nuances of terms like "disabled person" vs. "person with disability", see the resources at the end of this article.
1. Be creative, and as mentally and physically flexible as you can manage. Identify the things you like about specific difficult activities, and then actively seek out substitutes.
2. Communicate a lot, in and out of bed. Make sure everyone involved has the same goals, the same ideas of what works and what doesn't, the same ideas of why the things that don't work aren't working, etc.
3. Do a lot of research, from reading books and websites to talking with other people in similar situations. Fetish sites can be astoundingly helpful: for every disability, there's someone who thinks it's super-hot, from deafness to amputation. If you can get over being unnerved at having inadvertently become someone's fetish object, reading or watching porn that features people like you can be a font of useful (or horrifying) ideas.
4. Remember that being disabled can be really exhausting, especially if you're learning to cope with a recent injury or change. The world is designed for the able-bodied and neurotypical; those who fall outside of "the norm" have to endure a daily struggle to compensate. Aside from all the specific situations discussed below, the single most likely effect of disability on your sex life is that the person with the disability will be too tired for sex. Respect this and allow for it.
5. Don't harm yourself in the pursuit of pleasing your lover. Advocate for yourself, and if you need to stop or there's something you're unable to do, say so. If your lover says "I can't", believe them, and always be prepared to look for workarounds or just call it a night and try again another time. Disabilities are real. Take them seriously.
6. Remember that disability can happen to anyone, at any time. About 80% of adults in the U.S. will be disabled at some point in their lives, and that statistic doesn't include temporary disability from injuries like twisting an ankle or breaking a finger. So even if this article isn't relevant to you now, it almost certainly will be someday, either when you become disabled or when you get involved with someone who is.
Physical disabilities
A lot of sex is about leverage, since that's what lets you thrust. Disabilities that affect your arms, legs, or back can seriously interfere with leverage. To compensate, use physics, specifically gravity, momentum, and friction. A sex swing can amplify your motions so that a little hip-wiggling goes a long way. Wedge-shaped pillows can help you find the easiest angle for minimally painful exertion. Somewhat counterintuitively, bondage can be very useful for getting recalcitrant limbs to behave or keeping them in the least painful position, and if it hurts when you move a certain way, forced immobility can be a blessing. (Standard precautions apply, and that goes double if you have any issues with circulation or nerve damage. Play carefully and safely.) Friction is also what keeps strap-ons strapped on.
If repetitive motion is a problem for you, as it is for many of us who spend too much time online, turn to machines, which are designed for repetitive motion. Fucking machines aren't perfect, but they can be useful substitutes if you're having trouble doing your own pistoning or drilling. You can even (cautiously) experiment with making your own. Vibrators, particularly ones you can wear, also have a myriad of uses.
Sensory and neurological disabilities and debilitating illnesses, such as deafness, fibromyalgia, and multiple sclerosis, are difficult to discuss broadly because there's such a range of ability and each individual's response to it is unique. The important thing here is to communicate clearly with your partner. Especially in the case of illness, there are going to be "good days" and "bad days"; flexibility is key.
Mental disabilities
It's almost taboo to talk about adults with mental disabilities having sex lives, because in this culture, they're frequently treated like children (even those who are perfectly capable of running their own lives) and seen as asexual and undesirable. Nonetheless, it's quite possible for them to want—and have—fulfilling and happy sex lives. This is just as true for people born with Down syndrome as it is for those who develop autism in childhood or dementia in old age.
Compensating for the effects of mental disabilities on sex life is a highly individual thing; as always, listen to your partner and advocate for yourself. If communication is difficult, look for writing-based solutions if you and your partners are comfortable with them. Text can be a memory aid, which becomes a sex aid as well once you start remembering to go on the dates you've scheduled. Text is also useful for making sure that everyone has the same understanding of what's going on, where problem areas are, and how to deal with them.
Psychological and emotional disabilities
Mental illness can be tremendously debilitating. In addition, many anti-depressants and other medications used to treat mental illness can cause impotence or other sexual side effects. As with any illness, there will be good days and bad days, and something that's helpful one day may be useless or distressing the next. Say it with me: listen to your partner, and advocate for yourself.
Post-traumatic stress disorder (PTSD) can frequently interfere with sex, especially if it stems from sexual abuse or assault, which many people have experienced in childhood or adulthood. Sexual trauma can lead to distress in sexual situations (as well as seemingly unrelated situations like being tickled). The most important thing to remember about PTSD is that specific situations or behaviors can trigger a reaction that treats the present situation as though it were something happening in the past. To avoid triggering PTSD reactions in a partner, use the "enthusiastic consent" model of sexual interaction: rather than assuming consent if your partner said yes once, or failed to say no, look for signs that your partner is actively and enthusiastically enjoying what you're doing. If you don't see those signs, stop and check in.
If you're getting involved with someone who has a physical or mental disability, keep in mind that they're at least twice as likely to have experienced sexual abuse as someone non-disabled, and that the abuse most often comes from someone who has or had power over them (such as a parent or caregiver). Since sex is highly emotional and there can be a very real sense of power disparity if one partner has a disability and another does not, be prepared for post-traumatic stress to affect your sexual interactions. That doesn't mean that you should handle your partner like they're made of glass, though. For most of us, what hasn't killed us has made us pretty damn strong.
Many people who have experienced sexual trauma find they enjoy giving or experiencing pain, bondage, domination, and other kink activities, up to and including replaying the traumatic event. If this seems like it might be helpful to you or your partner, consider discussing and exploring it.
This really just scratches the surface of all there is to say on this incredibly complex topic. Below are links to informational websites, downloadable guides, online communities, and books that relate to various intersections of sex and disability.