I don't know that it can be forced per se. I think that if that's what you want, you just sort of need to get everything ready and then wait to be emotionally ready. I'm pretty sure pro-life literature reflects this as well; from what I've read they only really consider you "dangerous" if you express both imminent desire and describe having a means.
I've only been close enough a small handful of times (although in hindsight a few of those times my means would probably not have worked). I'm obviously still here (with some regrets but...), but when you're close you know it. For me, something needs to really set me off, beyond the normal day-to-day level of desperation/depression. Just my thoughts though, and obviously I didn't end up on the bus (or even manage to actually try).
One other thing I will say is that I've realized that this is a decision you have to make on your own. For the most part, people don't get to have any support with the decision. It's terribly lonely and I hate it, but these are the rules that the pro-life majority have imposed :(