Play in OTB tournaments. Read a book on openings, strategy, tactics, endgames, and an annotated game collection.
There are lots of books out there, but here are some to consider:
Openings or this one
Strategy
Tactics
Endgames
Annotated game collection
CT-ART is tactics software (basically a puzzle collection). Not necessary but it's fine if you like that sort of thing
Chess Position Trainer is software to help you drill openings (memorization aid)
An engine (which does the calculation and evaluation) and UCI (the interface that lets you use chess engines in general) are good to help analyze your games. Free engines here. (click on green or orange)
Free UCI here or here.
What would be the best way for an intermediate chess player to learn chess? What PC chess learning programs do you recommend and what books should I read?