i found a pretty good and very inexpensive training tool.
everyone has seen the "home gym" right? basically it's just two different length elastic? bands with handles that can be switched from one band to another.
they cost like $10 or something.
anyways, put both handles on one of band (depending on how much room you have) and wrap the center of the band around a doorknob or something secure.
stretch them out and work your punch techniques and combos. the bands provide dynamic resistance which maximizes at the intended impact point.
also, they will give you a decent upper body workout if you use them for a long enough time.