For some reason I've never been sure if it's correct to use 'to' with 'help'. For me it sounds more natural to say "Can you help me to do this" than without the preposition "Can you help me do this", which I think is correct as well.
Is it then possible to use 'help' both with and without 'to', and is there some kind of a grammatical rule behind it?
I usually delete it -- mostly because I don't like to clutter up sentences with words that aren't pulling their weight. But I always let "what sounds best" be the final standard.
If you use "to" in places that I don't, that is just a difference in our styles. There's a lot of room on this planet for a lot of styles; the more the merrier as far as I'm concerned.