Hi,
It is important that the milling cutter must cut rather than rub - the lightest cut you take should be at least 4-5 thou. and the feedrate should be steady and not be too slow.
There are two types of end mill - in the UK they are called End Mills and Slot Drills (in the USA the term End Mill is used for both types, but the slot drill is called a centre cutting end mill).
End mills (non-centre cutting) are designed for profiling (cutting on their side) not surfacing (cutting on the end) and you will get the best results if you use a slot drill for surfacing. If you have to use a non-centre cutting end mill for surfacing you will find the corners of the teeth are inclined to chip - this can be prevented by stoning a bevel on the sharp corner (it only has to be a very small amount, 10 - 20 thou or so, but make sure you maintain the clearance angle).
If you use a non-centre cutting end mill for surfacing, you will get better results if you take several narrow cuts rather than one wide cut - a good rule is to make the maximum width of cut 1/4 of the cutter diameter.
As for speed - I don't change the speed for finishing cuts, but the usual rules apply: Cut at a low speed and the job takes longer but the cutter lasts longer. Cut at a higher speed and the life of the cutter is reduced, but the job takes less time.
Ian.
P.S. It is, of course, possible to break all of the above rules and get good results. I.