In my experience, practice is by far the best salve for nerves. If you can give your talk cold, without slides, while being attacked by bees, a couple of thousand people won't bother you in the slightest.
Luckily, you're speaking in the best possible format for this. The neat thing about Ignite is the length constraint. With a five minute talk, you can practice properly (run through the whole thing, making notes on paper about changes you need to make) something like 6 times in an hour - that would take closer to 10 hours with a 45 minute talk. That said, don't work from a word-for-word script. Change things up a little on each run-through. That's always good advice, but it's even more important with Ignite, since it's easy to get off-track with the auto-advancing slides.