Go see a financial planner.. Lol..
Otherwise, i suggest you use some kind of procedure, mine is as follows:
If I dont purchase shares and the SP continues to climb will i be more pissed off than if i purchased the share and the SP retraces back to its low?
If answer is Yes: Buy the share.
If answer is No: Dont buy.
(Obviously this comes after extensive research, timing is a bit tricky)