Based on my experiences at the City of Portland, it seems there are a couple of things that should be best practices:
1) Mandatory Cooperative Purchasing Agreement Clauses- All contracts that governments sign should have clauses that allow any other government agency to buy off that same contract. It's amazing how many government entities don't do this.
Of course, awareness is another issue. Government entities have to have a centralized and well publicized place that they can find out who is buying what. The City didn't even put cooperative purchasing language in our standard contracts until 3 years ago. Ultimately this could mean that governments should stop buying technology on a organization by organization basis and start buying through a consortium. I know that many government entities want to believe that they have unique requirements for every other government entity, but that's usually not true.
2) "Flexible Goods" RFPs- We should have a way to buy any general type of software/hardware etc. as long as it meets certain functional requirements. This should also have some kind of rolling admission where if fail to apply during the initial RFP phase, that you're not locked out for a multi-year period. Companies should be able to get into the qualified vendor pool mid stream.