Connecting chords with inbetween melodies!?!?

Is there any good formulas or tips one can use to put melodic lines inbetween chords to connect them in a catchy rhythmetic way?

Yes. There are.

http://en.wikipedia.org/wiki/Nonchord_tone

This should help you out. Does a brief overview of the different kinds of nonchord tones (Like neighboring, passing, suspended, etc) that are used to "spice" up rhythmic pieces.