Work at the size of the avatar (best solution, but hard) or work with a bigger size. But what looks pretty when it's bigger, may look crappy when you resize it, because lots and lots of pixels will be thrown away or resampled, especially when you know that the avatar is only going to be 75x75 pixels (and that's not a whole lot). When you do plan to resize, use bicubic sharper as interpolation and you might even consider to slightly sharpen the final result.
I would sure not start with something bigger than let's say 200x200, but that's just my opinion.
That reminds me... I need a new avatar again (just got rid of the Halloween one)