I use DMC floss left over from my cross-stitching days. I'm often pleasantly surprised to discover how well colors of floss I bought in the 1990s coordinate with my SU cardstock!
I usually separate the floss into two or three strands when using it on cards.
Another thing I like to do is use two strands of one color and two strands of another color in the same needle. It's subtle but I like the effect. Here's an example:
dirty world by PH in VA
I prefer to use counted cross-stitch needles because the points aren't as sharp as embroidery needles. I use my piercing tool and a ruler first to make the holes for my needle, then go back and do the actual stitching.