I do as Rhonda K does - mostly. I use a larger than necessary ruler (cuz I can't be bothered with a bunch of specialty rulers unless really necessary). And for squaring HST's, always put the diagonal line down the center seam of the block. I also have a 12" rotating mat that helps with all the turning when trimming blocks.
I purchased several packages of rubber dots and put them on the corners of all of my rulers. Yes, they raise the ruler off the fabric a bit but they also don't slip around.