Start with
The Not So Short Introduction. Try and get started, and you'll quickly be able to tell if this is going to be reasonable for you to do or not.
You'll definitely want to have
The Comprehensive LaTeX Symbols List as a reference when looking for the various features of formulae. Many things are obvious and guessable (to get lambda, you type \lambda, to get multiplication symbol, you type \times) but others are stranger (infinity symbol is \infty, I have no idea why). Naturally, you quickly get familiar with whatever symbols you frequently use, and as a result, I think typing them within the document source becomes quicker than looking through a menu or what not to find them.
I'm sure it's not worthwhile for everyone, but it's free (open source), it's definitely a standard so it's good to know, so you might as well give it a try. It took me about a week to get relatively comfortable, but longer than that before I didn't have to look up all the various symbols or techniques I needed. It can be a bit difficult at first, because sometimes the behaviour of certain things can be confusing, and you'll probably end up with documents that fail to compile at first usually because of non-closed brackets and all, but like anything else, it should get easier over time.
If you decide to try this instead of buying a WYSIWYG-style math program, feel free to ask me about any problems you may encounter.