Keyword research is not something that should ever stop, it needs to be something that you continue through out the life of your website.
The key words are the life blood of your website and ensuring you have enough relevant and commercially economic phrases to look at in the future will ensure you do not get to a point here you are unable to grow and your site becomes static.
There are a number of tools available to help with your keyword research, among them is Googles free keyword research tool, it gives good general information about suggested words to target, the levels of visitors and some idea of what you would expect to pay for a Pay Per Click campaign.
The number of keyword research tools is growing and the only limits are your budget, most good Search Engine Optimisation (SEO) companies subscribe to 3 or 4 paid for services that can cost several hundred pounds a month, but as the research is important this is not a cost that should be scrimped on. >
Looking at the keywords and phrases that your competitors are ranked for is a good way of learning the phrases you need to consider and using some of the more advanced features of the paid software you can drill right down into this information to give you the competitive advantage you need.