Well, to get a sharp and clear scan, you need to start with a sharp image. There is a reason why some 100-300mm lenses cost $150(US) and others are $2000: image quality. No, I'm not saying that you NEED one of these fancy "whiz-bang" lenses to get pics accepted here (most of my stuff is shot with a peice of sh*t Canon 28-85mm)...I'm just saying that it helps a TON.
Once you have a sharp image, you can get a clean scan with pretty much any decent scanner (though scanning a negative or slide will provide the best results). The key word here is PATIENCE. Some people just scan the image, save it as a .jpg, and upload. There is SO much more to it than that!! Sharpening, adjusting color balance and levels, cropping...it all takes time. Photoshop 5.5 is the best tool out there for this kind of stuff...but any good photo editing package will do. You'll be doing yourself a HUGE favor by learning one of these packages inside and out. Patience, patience, patience. In my opinion, getting a sharp picture is only 30% of the battle...post-processing is where the magic happens.
Anyway, yeah...I too am amazed at what some people are uploading to this site. Every now and then, as I sit and browse through the new pics, I find myself mumbling something like, "how in the world did that guy DO that???!?!?!" There are some great photographers here, and I feel like I've learned so much just by closely examning their pics and comparing them to mine.
San Diego, CA