First, make sure your ramp isn't too steep. Mine is two bent grids zip-tied together, which provides an adequate angle (at least my piggies think so).
As for footing, I have a coroplast base sitting in the grids. The coroplast is then covered with shelf liner (it's a sort of rubbery material). I was hesitant at first because I thought they might get their nails caught it in, or chew it up, but neither of these have happened. Works great! A roll of it was 94 cents at Walmart.
Fleece doesn't provide very much traction for piggies. I use a towel with big loops. It's not a very soft, comfy towel, but it provides good traction for my piggy. I had this at home, so it was free! I use binder clips to attach the towel to either side of the ramp.
You could also do what WildRye suggested and buy rubbery shelf liner (which is cheap). Many people on this site use that.
Some also use a piece of carpet. That might be a more expensive option unless you have some carpet lying around.
We used strips of denim for our ramp. If you have some old jeans lying around, you could just cut out a strip or too. We used double sided tape to attach it to our aluminum ramp. The only problem with that was that it got nasty when they peed on it. So, if I were you, I would clip the denim to the ramp.