I found this great idea while ago in Bob Walsh’s book “Micro ISV: From Vision To Reality”: keep a summary of all decisions you make in a “Decisions Document”. Bob was talking about it from the point of view of a one person start up (or micro ISV), but it can apply equally well to a software department. This should be done for each project individually as well as another document for the department as a whole.
The Decisions Document should include a short description of the decision, the reasons for making it, the date, references to any documents or research material, and where appropriate the people involved in making the decision. It allows you to re-visit decisions at a later date and re-assess whether they still apply.
Design is not just about what you put into an application or product; it is also about what you decide to omit. An excellent place to keep your decisions document is in a Wiki (along with your other development artifacts).