I always try seeing what they look like with the PNG-8 setting in Photoshop first. If it sucks, I try setting the # of colors to 256. If that still sucks, I just make it a JPG, unless it really needs the transparency. Generally, unless it’s a giant background image, if it’s over 150k or so, that’s way too big. Random posts in blog images I try and keep under 50k.
This is a good tool for getting the most out of PNG’s http://pmt.sourceforge.net/