Long time lurker - infrequent poster...been kinda swamped. Here's some bad guys I doodled once for a hobby project.
Arc, as long as it's fun, just keep noodlin. Improvement is inevitable - if anyone spends enough time at something they're bound to get proficient.
For instance - it's apparent that I don't spend a lot of time drawing city streets judging by these doodles!

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Love your work. The first one is like the cover of an old sci fi magazine, the design of those extraterrestrial faces is just awesome. Never thought rock texture and diamond eyes could be so expressive. The litterbug and and the Sponge seem to me like very good ideas, and the design is amazing too (I especially like the Sponge, it's quite imaginative).

And yes, I agree, practice is always a good thing.

Darren: I do have a good time sketching. I wouldn't keep doing it if it wasn't fun for me (after all, it's obvious I don't make money with my sketches).

And here's the last one: