Making 808 samples sound better involves two very important factors. First, we have to adjust the Attack, Sustain, Decay, and Release settings in order to make the sample match your tempo. Secondly (and arguably more importantly), is playing with the EQ tools within your sampler or program. Unlike a guitar or a synth sound which has its fundamental frequencies set out front and relying on other frequencies as supplemental space fillers, the 808 sample packs sound like a giant mess of clattering metal scrap all mashed together into one huge soundscape that's hard to focus on any one individual frequency. The best thing you can do is use subtractive EQ by taking away all of the lower frequencies so that they don't muddy up the higher ones.