this was a problem for me too, what i ended up doing was a combination of finding the right height to have my guitar strap as well as doing some of my practice time standing up.. i'll sit and play some scales, then stand up and do some other exercises, just alternate standing and sitting every now and then and it becomes very easy...

but i'd say finding the sweet spot for where to have your guitar is the most important thing.. and make sure when you practice sitting down that you don't slouch over the guitar too much because that will make a bigger difference too..