I've been mixing some songs for my band and doing some really basic mastering techniques on the mixdown (i realise that mastering is something that professional engineers pick up over years of experience, but I'm just doing some very simple things like overall EQing and multi-band compression). I'm finding that when I listen to it on Pro Tools it's at a good, loud, strong volume and isn't clipping, but when I bounce it down and play it back on itunes it's a lot quieter and lacking the impact that it had before.

any ideas?
That happens to my songs too. Just insert them in your programm again and make it louder

i should also say that im using max limiters AND normalizing to try get it sounding big...the song i was working on didnt have a big dynamic range so i felt it had to be like a fairly typical mix like you would hear on the radio, compressed to hell to squeeze the life out of it. and its still kinda quiet.
heres what you should do: not care.

i know, it doesnt make sense. but it will in a second. the music that is produced today is limited to hell and back, so it sounds way louder. this means that the dynamics are all but shot. i know you said you compressed and limited it, but the guys doing for stuff these days are pros at squashing those audio files. they have the experience and equipment to squeeze every last drop of volume out of what they are given.

if you just turn your mix up, it will probably sound fuller, since you havent killed all the dynamics anyway.

if you really want to go ahead with the max volume, compress, limit and normalize each track (or at least your bus/group tracks) and then do the same with the overall mix. the stuff done in studios is done in stages, not all at once.