MySQL indexes - what are the best practices?

I've been using indexes on my MySQL databases for a while now but never properly *learnt* about them. Generally I put an index on any fields that I will be searching or selecting using a `WHERE` clause but sometimes it doesn't seem so black and white. **What are the best practices for MySQL indexes?** Example situations/dilemmas: * If a table has six columns and all of them are searchable, should I index all of them or none of them? * What are the negative performance impacts of indexing? * If I have a VARCHAR 2500 column which is searchable from parts of my site, should I index it?