if you don't fancy a NW chainring (availability, cost, or you want an odd number of teeth)…..
- a 3/32" chainring meant for fixed gear works better than one for a double-ring derailleur setup, because the teeth are full height.
- a chain with cambered inner side plates helps the chain stay on (not all 8s derailleur chains are equal in this respect)
- if you take a 1/8" fixed gear chainring you can file the teeth so that they fit the chain (i.e. are about 2.2 to 2.3mm wide, which is wider than a typical 3/32" chainring; they can taper slightly too) and the chain will stay on better than with a 3/32" chainring. A potential downside is that if the clearance is too small and there is too much dirt, the chain may start to bind on the teeth (so give chainsuck type symptoms). This approach works really well on a road bike with mudguards though, and the chainring is really hard-wearing because it is wider than anything else you can use with that chain.
BTW a clutch-type RD helps too.
cheers