Most people find a higher table is easier. When you have to bend over, it's not only hard on your back, but you have a tendency to lean on the ruler, which causes it to move.
If you work with a sharp blade so you don't have to push as hard, it's easier to keep straight.
Fold your fabric so you don't have to cut all the way across that width. I usually fold into quarters (as it comes off the bolt, then bringing the fold to the selvedges) so I don't have to cut as far. It's easier to keep straight that way. Also, work with your fabric close enough to you so that you don't have to reach for it.
Press down on your ruler, FLAT, instead of pushing or leaning on it. I hope you can picture what I mean by that. Straight down.
I prefer Omnigrid rulers and can't stand rulers with stuff on the back that prevents me from manipulating it into place. I took a good class from Debbie Caffrey at Quilt Market a few years ago. She supplied us with Omnigrip, which is like clear "window cling" stuff. You know those peelable seasonal decorations? Anyhow, you just stick it on your ruler (it just "adheres" - it is not sticky) and then you can slide your ruler around all you want. But the instant you press DOWN on it, it STICKS to the fabric. This stuff is EXCELLENT!