I've done a few and have had the best luck with three electrodes: one left, one right, and one ground in between.
If memory is serving me correctly: an instrumentation amp with a gain of 50, a band pass filter with 12dB/octave roll-off outside of 45-290 beats per minute, and a final output amp/buffer with a gain of 10. Strong hearts can send the output from positive saturation to negative saturation with something like an LM324 connected to 5V supply. Rail to rail op-amps, lower gain, or higher supply voltages can be used to eliminate most of the clipping but that is unnecessary if you're just detecting and counting heart beats. Weaker hearts will end up with about 1.5 Volt peak to peak signals.
Three properly placed electrodes allows you to more effectively cancel out common mode noise and concentrate on the differential mode signal. The result is a S/N ratio that's a few decibels higher and with these circuits those few decibels can be the difference between working well and not working at all. Even with the third electrode, the processor will still be analyzing a noisy signal.