I've been bitten by this too many times. You find a well known hosting provider, read comparative reviews (and pass by on any company that has influence in those review), they promise 99.99% uptime, you pay a reasonable price (not the cheapest) and after you bought the server or VPS, they end up delivering less than 95% uptime. I'm not naming any names, but these are some well-known companies.
How can you independantly verify the validity of their uptime claims before handing over the money?