For a student, what is the optimal usage of a solution manual?
Never use the solution manual before trying everything else; talk to friends, visit the professor, go to class and listen (!), check the internet. Once you use the solution manual for a problem, the potential gain from that problem is significantly and irrecoverably reduced.
Use the solution manual to check your work. (Duh.)
For problems you aren't planning on solving, you can use the solutions manual to create flashcards and other learning aides (if the course material is anemable to such a construct).
If you have a friend/roommate/spouse/trained monkey who can compare your answers to the manual for you, such that you don't actually read through the manual, that may be useful for certain topics.
You can make some good money selling it when the semester is over :)