Tension on V-belts can be a little tricky. There is a tool to get the proper amount of tension, but I have found that the cheapo version of the tool doesn't work that well. A good rule of thumb is that you make the belt nice and taunt without levering the accessory drive too much. Take it for a spin and make sure you turn on the accessories. If is squeals, give it a little more tension. Too much tension can wear things out prematurely, like the bearings in your alternator. What you are looking for is a certain amount of deflection in the mid-span of the belt with a given amount of force applied. Do it a few times and you will get the feel for how tight to make them.
Also...the sheaves that the belt run in (AKA - pulleys) can get burnished from too much slipping. If you have a belt that just keeps slipping no matter what you do, including putting new belts on, it is likely the belt surface of the sheave is burnished and the only way to fix it is to put a new sheave.