Maybe i'll be telling my own opinion, but.
Look at my replys at graphics forum. count the games that were not abandoned. It's not even 5%!
When you think about it, you realise that you have wasted your precious time, that could have been used\wasted for
anything else.
So, normally, I'll help in case when I already have that material done (remember these numerous topics with games that need a nice loopable sky background?), or I remember topic author, or I see several completed games in authors sig (the more games you finish, the lower are chances to abandon your current project, even while for some people chances change slowly).
After some time, you may be able to detect potentially dead topics, and compare your own topic with them.
Also,
* never ask for huge photorealistic images, noone will do them (these topics are never solved and rarely have over 2 reply's).
* never ask for ANY anime-related spriting (after posting a topic requesting for 64x64 or above naruto sprite, you can just go and kill yourself over nearest wall. these topics are never solved).
* never ask for huge animated character spritesheets. people will never do 20+ frames just for a credit.
* before posting a topic, look at it: would a same topic by other person motivate you to sit down and sprite?
Edit: what am I talking about analyzing:
452457 [-] a lot of requests. not even much people have read it all. but attention: renders can save him.
451754 [-] check it out: he combined nearly all possible mistakes into one topic
452477 [+] he\she asks ony for single images, which are easy to do. request will be solved today-tomorrow.