And yet another opinion chimes in:

Unless you think of a very obvious advantage in having or not having multiple payment periods (such as having only semiannual or annual on your least-expensive plan, to try and weed out the scammers), you might want to think about offering monthly, quarterly, semi-annual, and annual - or at least the first three. It gives the customers the choice, which is always good - especially if the customer sees 'Oh, look, I can get a slight discount/have fewer things to remember if I pay once a year, but I can choose to start out with monthly payments.' or 'I prefer monthly payments, but I know that the next quarter's going to be lean so I want to consolidate payments where I can until I get through this rough patch/busy spot.'

75% of customers will not change their payment cycle once they've chosen one. But almost all of them will appreciate having that choice available to them. It's almost as much a psychological as a financial question.

I haven't seen too many hosts offering multi-year payments; but I suppose you could offer that to longer-term clients who ask. Just keep in mind that your income will be a bit more spastic. From your side of things, you'll want to try and have a fairly equal income-curve from month to month. It's a moving target, but there are definite advantages to knowing that you'll always have at least X amount of income each month.