1) Short and sweet: put solutions in the puzzle thread hidden by a spoiler tag
2) Long and bitter: the puzzle poster should create two threads, one titled "puzzle name" the other titled "puzzle name [solutions]". If you are the first person to respond to a tricky puzzle and it doesn't have a solution thread yet, make it yourself. Please remember to put a link to the other thread in the first post of the thread (ie in the puzzle thread put a link to the solution thread and in the solution thread put a link to the puzzle thread).
Ask questions about the puzzle in the puzzle thread, put answers in the solution thread un-spoilered (that way there will be less possibility of people arriving at the same solutions whilst percieving their contribution to be unique and we can also discuss whether a particular solution is a genuine solution in plain text the way nature intended us to participate on forums).
Feel free to discuss these policies here in this thread. If I like what you're saying I'll change the policy, but I think this will work well.