You can try a mantra that is more rythmic. You don't have to do japa of a formal mantra, instead you can do a hybrid of kirtana and japa. This is a better method for most people. This mantra is great for this purpose.
राम राघव राम राघव राम राघव रक्ष माम
rAma rAghava rAma rAghava rAma rAghava rakSha mAm.
कृष्ण केशव कृष्ण केशव कृष्ण केशव पाहि माम
kR^iShNa keshava kR^iShNa keshava kR^iShNa keshava pAhi mAm..
When doing dhyana it is better to use descriptions from shastras to visualise. Look at chapter 99 of the patala khanda in the padma purana. Here there are 22 dhyana shlokas for Krishna you can use before doing japa or archana.