How do you determine if an Agile, Waterfall or hybrid approach is best for your project?

Looks like most commentators agree its about choosing the right approach for each project. One thing I've noticed in recent years is a growing awareness in the industry that the various methodologies, approaches and tools are just that - things we can use to make sense of a project. I've found, as my experience grows, I make judgement calls as I go about which to use. My bottom line is doing the least I need to do to run a project safely as most people around me aren't project trained so find the process and jargon a barrier. I tend to plan in stages and apply an appropriate approach for each stage - whether 'waterfalled' or 'iterative' or a bit of both.
I've also noticed PMI's recent work in the soft-skills space and I've found that no matter what tools, plans, documents etc I use, its all about the people and relationships. I've worked with thorough project managers who struggle to relate well with others, and so depsite having a well-planned project they have problems delivering.