1. Research and check all available facts. Look for real statistics if possible: What happened to most other folks, will likely also happen to me. I'm not exceptional.
2. Are there any distinguishable patterns? That is, does this situation resemble another situation that could be used for comparison?
3. What are all the likely outcomes? If I choose X, what will happen? If I choose Y, what will happen then? What other choices might there be that I'm not thinking of? Make sure all options are on the table. Try to think ahead several steps. Example: If I choose one research area for graduate studies, what will happen if it becomes unfashionable in 15 years? Might there be another choice that would allow more flexibility? I tend to choose things that leave the maximum number of options open, whenever possible.
4. If the choice that seems best now suddenly goes bad in a hurry, how can I un-do it? Most choices, no matter how irreversible they seem at the time, can be un-done. The only real exception is having kids, you're stuck with those for life. There should be an exit strategy that is feasible and that I could carry out.
5. Consult any experts: lawyers, financial advisers, doctors, etc.
Usually in that order. Experts are expensive, I don't like to go to them without some very clear ideas. The big mistake I have seen many folks make (including me) is the "it won't happen to me" fallacy--they reckon that they are special and that even though everyone else fell to their deaths when walking on the cliff edge, somehow they will be OK. Then they are shocked, shocked! that they are lying on the rocks with broken bones. Particularly if obtaining a thing you want involves a "tournament" model to achieve it, or that is extremely competitive, for example playing sports: most college sport players will not become major leaguers, so they need to have a backup plan for a job post-graduation. Yet I used to see quite a few kids in college who felt that since (sport of choice) had gotten them a full scholarship and other amenities, therefore they did not need to study.
I believe it is a very good thing to think about decisions long and hard before making them--too many people do not think at all, and then expect you to feel sorry for them when everything goes to heck. Just my opinion.