I also have trouble with the dreaded V on the fold most of the time. If I take the time to find the straight of grain by ripping WOF and then line up the selvages; I usually do all right. Some fabrics seem to be too warped no matter what and then I just cut parallel to the selvages instead of WOF.
To keep the ruler from sliding I have put sand paper dots on my small 6.5" square ruler that I use when squaring up patches or fussy cutting. When cutting WOF, I use my 6 x 24" ruler and keep my pinky & ring fingers on the matt to steady the ruler & slide my hand up it if necessary; pausing with the rotary cutter while I slide.
I keep looking at the June Tailor Shape Cut Plus ruler and daydreaming but I have the Alto Quilt Cut and need to make more use out of it. It will hold the cutting guide & fabric steady but I normally only use it for cutting scraps into usable sizes. I got in the habit of that even though I know it will work for yardage and just haven't forced myself to do it.
Others seem to like using the suction cup handle things on their rulers. There is a thread on this board about finding them at Harbor Freight for much less than the quilt shops charge.