Stolen from the bitcoin forums. It's real handy to have a cheat sheet for bitcoin mining flags.i.e. I've used -w256 -f60 in the extra flags field and didn't understand what the heck I was doing until now.

-v or --vectors sets it to use vector calculations. in general, this is only a good idea for newer ATI/AMD graphics and gives you a fair speed up. it slows things down on nvidia cards.

-w or --worksize controls the work group size. it defaults to the maximum value reported by the card and going above that will likely result in the miner crashing. there's no real hard and set rule for this and it varies from card to card, so find someone with your same card and ask, look at the mining hardware comparison for inspiration, or just poke in various values and look what happens to the hash rate.

-d or --device chooses what device to use. this is only needed if you have multiple graphics cards

-f or --frames controls the execution window. basically, this is the priority settings and higher number=lower priority. setting it very low (1 or 0) will give you a bit faster hash rate at the cost of slowing your desktop to a crawl, which is fine if you're leaving it unattended for a long time. setting it to a large value (hundreds to thousands) may allow you to even play games while mining in the background, depending on your hardware.

-a or --askrate controls how often the miner sends a getwork request. you should use whatever value your pool suggests or just leave it at the default if solo mining

-p or --platform controls what platform is used. only useful if using multiple graphics cards from different manufacturers (nvidia vs. ati/amd) or if you're using the GPU miner on your CPU (which is a bad idea, as it's much slower than a proper CPU miner and CPU mining is at best barely worth it)

Source: bitbash.blogspot.com