I'm a fairly new quilter, and I'm very familiar with the crooked middles of strips. I've figured out that it's caused by the ruler moving slightly as I cut. I try hard to put enough (but not too much) pressure on the ruler, and enough (but not too much) pressure on the rotary cutter. I also find that the sharper the cutter blade is, the easier this is.
I read somewhere that some people glue little squares of very (*very*) fine grit sandpaper to the bottom of their ruler to stop it slipping. I think I've also seen some sort of "grippy dots" for rulers in quilt shops. I should probably try one or the other!
Jan