How do I make a video crossfade/dissolve between two clips?

Kdenlive 0.7 does not have a video crossfade/dissolve transition by default.

You can either use a "composite" transition or a "luma" transition to achieve a crossfade.

For a luma transition, just use one that "looks right". With composite, you can get a real crossfade/dissolve by applying a composite transition, then add a keyframe to the end of the transition and set its opacity to 0.

For some more discussion of the composite transition, please see mantis issue 261.