I first bought the little clear dots and also the sandpaper dots. They works well but what I did not like about them - with the clear dots - the rulers edge was not completely flat on the fabric while the sandpaper dots were always "blocking" something that I was trying to see.
Which leads me to my most recent find... Rubber Cement (OK before you international folks ask - I am told by others that in some places it is not easily available outside the US). I went to a Machine Quilters Expo in Providence RI and had taken a class on longarming with Dustin Farrell and he suggested using it. I tried it and love it. When it gets nasty with thread it rubs off nicely and gets a new coat. You can see through it, the ruler lays flat and grips perfectly. I apply it and let it sit until it drys before using the ruler. AND did I include cheap?? LOL