As someone else said, suspenders are probably your number one bet. Any of the commercially available fanny packs have suspenders available as an option, so check to see if yours does. As I do not know what kind of pack that you have, I can't really help you with that.
Around here, pretty much everyone uses the Traverse Rescue Cirque 2, http://www.traverserescue.com/store/...ctid=239&cat=0
and most of us have the suspenders. http://www.traverserescue.com/store/...ctid=190&cat=5
Just to be contrarian, I have a Mountainsmith Day, http://www.mountainsmith.com/product...bCategory2Id=0
and I also have the suspenders. http://www.mountainsmith.com/product...bCategory2Id=0
The great thing about suspenders is that you can really load down your pack, and you barely feel the weight.
Someone else also talked about ensuring that the straps of your pack are properly arranged. I would also tell you to ensure that your pack is properly adjusted. If you have a decent or high quality pack, and it is always slipping, I am ready to bet that one or more of your straps are improperly adjusted, making it is impossible for the pack to stay in place. If you are not sure how properly adjust your pack, visit a good hiking/camping boutique and ask one of their people to do it for you. Chances are it would help. Remember that the adjustment methods on some packs is arcane to say the least!
Finally, if you have a really lousy pack, invest in a better one. The Mountainsmith that I carry retails for around 80 dollars, but I got it for a bit more than half of that directly from Mountainsmith. (PM me if you want details) A good pack is so comfortable and you use it so often that it will pay for itself in a few weeks. But remember, if you bought a ten dollar bargain-bin special, you will be paying for it in discomfort until you replace it.
Hope this helps,