For general purpose, I used to reset AT89C51 and AT89C2051 as follows:
RST -> 47uF (to +5V)
RST -> 15K (to Gnd)

But for reliable reset, I use an NPN transistor (for example BC547) as follows:
Base -> 12K (to Gnd)
Base -> 4u7 (to Gnd)
Base -> 68K (to +5V)
Emitter -> Gnd
Collector -> 22K (to +5V)
Collector -> Reset pin

Kerim