1. More accuracy with your strikes so you hit with the correct striking area and with good technique, that will lead to less bruises.
2. If you don't want your bruises to come then out ice your boo boo, always use ice wrapped up in a towel or something. Bruises are souvenirs that you can look at and poke your finger into, they are part of developing mental toughness. It will help you deal with the pain of actually hitting someone for real.
3. Rest the bruised area before breaking with it, you can use the palm instead of the fist for example, don't try to do too much in too short a period.
4. Conditioning, both mental and physical, remember every minor injury you live through makes your mind stronger.