Absolutely the best way and time is before you have made a commitment...however that's not always an option! I wouldn't try to tell your partner that you are poly that can lead to all sorts of hurt feelings, and questions you really don't have an answer for. The really great thing you can do is read up on polyamory and what it takes to have a really strong poly relationship and work that into your current relationship first. Build your foundation for later! Get used to talking about everything and anything because you will need those skills for later. Also be prepared to hear that you have broken trust with your partner and that they are not prepared to accept your announcement. Can you live with a firm no? If not then break off the relationship and be honest when doing it. No is always a consideration and coercion doesn't work in the long run.
Yes could also be an option...how will you handle a relieved partner saying, "FINALLY!!! I have been waiting for you to say something?" Will you suddenly have those feelings like you don't know your partner? It's something to think about.

At any rate I would move slowly and start by discussing the poly lifestyle as though it was happening to someone else to take the pressure off. Ease into the discussion because lemme tell ya, "Hey I want to watch you sleep with other guys and I want to as well!" or "I think it's possible to love more than one person at a time and I want to explore that with you!" Is a shock and can really really cut deep into the trust you build as a couple. Unfortunately the assumption is still that each man will find the perfect woman for him and vice versa and that's all you will ever want and need. Challenging that belief can be horribly painful...but in some cases is totally worth it.
Most of all I advise being gentle with your partner and with yourself, this isn't an easy decision but it can be a freeing one.