https://www.lesswrong.com/posts/aiz4FCKTgFBtKiWsE/even-odds is great for actually setting the size of bets.
Free-form betting typically has rather high transaction costs. Formulating a bet in such a way that all participants will agree on the mere proposition itself is time consuming for at least one party and also often demands substantial coordination from both or all of them. Sometimes it also requires attention from an independent arbitrator. Disputes over resolutions of bets can cause vastly more loss of value from all participants than any possible undisputed outcome would have.
Furthermore, as soon as you bet nontrivial amounts of money, incentives start to favour some form of shenanigans on the part of those offering the bet: concealing technical loopholes in the statement or resolution criteria, misleading other participants about what the proposition actually is, or being able to affect the outcome in some hidden way. Protecting against any or all of these is not trivial.
My opinion is that in short, trivial bets aren't worth the time (except for purposes of practice in probability), and nontrivial bets are risky for reasons far beyond the question that is putatively in doubt. I would mostly leave it to risk-seekers and scammers.
For our November meetup, the Chicago Rationality group is focusing on predictions markets, and more generally on the practice of betting on beliefs. As part of this, we have a "homework assignment" for the meetup: each of us is trying to make at least one bet before the meetup happens. This turns out to be kind of hard! I've challenged a handful of people on some online forums to bets and found no takers. Unsurprisingly, when people make absolute statements and you ask them to put money behind them, they become much more circumspect (which is kind of the point).
We're looking for people who want to make direct bets, individual to individual. Ideally, these bets should be:
Why are we doing this? Because I think rationalists should be betting more often, and betting more size.
Finally, I encourage people reading this post after any bets have settled to not judge people on their wins or losses. Bets that have a high expected value can lose, and bets that have a low expected value can win.