What are general rules when deciding on index?

If selectivity of condition is low (almost all rows match) – there is no point in putting the column in index
If you have condition that you “always” use (or, at least very often) – you can put it in WHERE part of index definition
Don’t add too many columns to index – it will make it larger, and less efficient. If you can get 2 columns to match 1000 rows out of million, and adding 3rd will reduce the match to 900 – don’t add the 3rd – it will be, most likely, better in the long term to do index scan on these two columns only, and filter the rest normally.
Remember that generally speaking you can have only one inequality condition in query being efficiently handled by index scan
Remember that any columns in index after column used for inequality comparison, will be most likely not compared against index, but rather will be filtered out