Watch the Hammer-ons Breakdown online guitar lesson by Ben Lacy from Two-Hand Groove Guitar
When I hammer-on I press close to the fret but not right over it. Press firmly but don't kill it.
Use your left index finger to keep notes from ringing on. After I hammer-on with the index, I leave it on the string to mute it. After I hammer with another finger, the index mutes as well.
Spice up your bass lines with vibrato and double notes. Playing double notes with the same finger is nice and subtle, while using two fingers gives a more even and pronounced attack.