If you havent't read tip #1 (take a Day Off), go and read it now.
TIP #2 Make a friend.
Some problems are seem much more challenging to solve and sometimes may even seem impossible. If tip #1 hasn't solved your mental block then its time to move to tip #2, make a friend.
This is a simple exercise where you pair up with someone you know and trust. You explain to them the exact situation (being as brief and concise as possible) and then you ask them to explain the situation back to you. You may realize that even though you explained it as clearly as you can, they end up with a slightly different understanding. This slightly different reality may be enough to jiggle your creative thinking by allowing you to see the problem through the eyes of someone else.