Finally decided to take a look at these. Constructive criticism?
The art is very simple, but that's alright - comedy strips don't necessarily have to look like anything, and as long as everything you create sticks to one theme, you're golden. However, since you only create heads, the options you have for characters is pretty limited; they're probably going to be doing a lot of simple talking. Which brings us to:
The writing. This could use some work. Because of your characters, your strip relies COMPLETELY on this. No slapstick, no situational humor - just dialogue. That means your scripts need to be frickin' spectacular. Try to condense your strip into three panels; that's enough for a setup and a punchline (which is all you need for the snappy one-liners you've been taking shots at). Cram all the filler in to that one extra panel. If you can't, you might as well go for more than four (but how many you should use really just depends on the joke).
If you laugh at your own strip, you're on the right track. Oh, and break the fourth wall. That's always good.