I've done a fair bit of this in the last year and the solution I've come up with (which may not be the right or only one!) is:
1) Cut to >95% of slot width with a roughing cutter - these generate lumpier chips which seem to clear the slot better - then if you need a clean slot do a thin finishing pass with a "smooth" cutter.
2) Use a cutter that is <66%of the final slot width to aid chip clearance.
3) I use (generous) mist coolant with a fearsome air blast - if your slots are annular or change direction multiple nozzle will help.
4) Play with feed rate to see how hard you can push it - with practice you can judge quite a lot from the noise IMHO.