I've eased myself into these waters by investing in an existing business. I know the owners well, and they are good (e.g. ethical), hard-working, committed folks. It's not in my field (research) but it is in my industry (energy). I'm providing advice/guidance on strategy, financing, etc., while keeping my day job in the hope that the business will grow enough to give me the option to work there full-time. I structured my investment as convertible debt to limit my downside, give me the equity option, and help this business at at critical time when its double-digit growth is creating a significant working capital crunch.
I've also been involved with two start-ups that didn't make it. In both cases, it was not accounting that did us in, but the leadership. Specifically, the leadership provided by big vision entrepreneurial types that, while providing a valuable service as head cheerleader/morale booster/enthusiastic-promoter, should not be let anywhere near the operations side (e.g. how to turn the vision into something that actually makes money).