Get there as early as you can and don't let anyone nudge you out of your place - if anyone pushes you, you push them back.
I've been to a number of premieres and it's a fantastic day out - but be warned it's also a very long, very tiring day. Make sure you have access to food and drink and, if possible, something to sit on. Take warm clothing and an umbrella at whatever time of year because you're outside for hours and will possibly need to wrap up at some point.
Be prepared that you might not manage to get autographs even if you're at the front - it's totally down to luck whether the stars come over to you or not no matter how good your place is. If you're going with the desire to see a specific star try and see if you can find out beforehand whether they're going to be there because it's rare for an entire cast to attend; there are usually at least a couple missing.
Most importantly, have a good time. It's a very different kind of day out and not for everyone, but I think well worth a try if you haven't done it before. It's an interesting experience whatever happens and one thing I like about premieres is I always end up making a few new frienRAB each time.