Another critical thing is really good hold down. Movement can destroy the piece and likely your end mill. Even very slight movements due to vibrations in thin metal for instance, can cause a very poor cut and a lousy surface.

I use the single flute "O" types too, and I've had very good luck with inexpensive pure carbide single flute no-name end mills purchased for $1.25 each on eBay. The smaller end mills seem ok for cheapo. For larger ones, I'd go with name brands. I have a couple of those 1/4" Onsrud 63's myself. They work very nicely.

I go with lower RPM's rather than higher. I like 6,000 for 1/4" single flute O's, and 8000 RPM's for the 1/8" end mills. Using a slower cut reduces the deflection in the machine and for me, makes a nice surface and a closer tolerance.