This is something that I touched up on here, whilst discussing whether a theory that I made had enough evidence to make it liable.
Here is what I think is needed to make a good theory:

• An idea to base it on - This usually involves the scientist or philosopher researching something beforehand, or making specified observations. The problem I found was that this is quite often what is later used as evidence for the theory, even though it is the information that the theory was founded upon.
  
• Evidence - Without evidence, you have nothing to back up your claim, giving people little reason to even consider it. To get this evidence, often peopel carry out experiments to work out, firstly, the specifics of their theory and, secondly, whether these specifics fit it and are compatible with basic factors that are already 'known'. This is a term used lightly as, also touched upon is the fact that nothing is proved, and so what we 'know' is in fact, generally, a set of commonly believed factors that are extremely likely and usually have more evidence supporting them than against them. These factors are usually very basic science, such as what we believe is 'physical' and the sorts of things that we base other factors upon, like the 'fact' that we live in 3 dimensions.
  
• Time - This is something that I only really thought about today. The effectiveness, in terms of how much it is believed, of a theory depends on at what time in histroy it is suggested, or thought about. For example, religion, as a whole, was 'created' a long time ago and, back then, more people believe the ideas behind it than do today. If the theory were to be suggested today, would it be accepted? Surely it contradicts a lot of what science had proven, and so, in this case, would be less likely accepted.
  
• Something that is, to the least extent, possible - This is quite a strange point, as you could suggest that anything is possible. Although, the more likely a theory is, usually the more accurate it ends up being. This does tie in with the evidence, as the more evidence provided for the argument does make it more likely. However, if this thoery defies even the simplest laws of science, then it is very unlikely to be plausible. On the other hand, making a bold theory that goes against everything that we believe in could, in fact, open our eyes to a new way of thinking. It might create a different set of scientific laws that could, in some instances, still work. Wouldn't that be interesting?
  

At least, that is what I feel makes a good theory. What do you think is needed to make a good theory? Is it something that is purely 'scientific'?