I've built 6 story buildings for my Base. One of them is Grape Tower, which is sloppily held up by chains, dinosaur heads, thick columns, spit, and luck. That was how I built 4 years ago. Here is a technique that I use now:
Square first floor, usually 2 studs thick but it can be any thickness. Then either I put plates on the top if the building's small, or big thick plates if it's a big building. If you want extra stability, you can double or triple stack the plates, OR put cross beams from the top of one wall in the building to the opposite one. There are SO many different ways to build buildings. Here is a diagram: EDIT the diagram is messed up when it posts nevermind.
If you want really good in depth but easy building techniques and stuff you could get 'The Unofficial Lego Builder's Guide'. It's a great book, it has all kinds of awesome essential stuff in it.