It's recommended that you tweak your search tool to recognize misspellings, synonyms and even terms of products you don't carry so you can show similar results.
For example, one of the top 10 best paid search terms for Walmart is "prevacid." It's likely a very common search term on the site as well. But if you misspell the anti-indigestion drug, you might choke on the "zero results found" message you receive:
If many people are searching for "prevasid" on the site, that could turn into a lot of lost business for Wally-mart.
Trouble is, there's no automated way to do this - there's no universal thesaurus that can predict all the terms you should include in your search dictionary that considers brand / product names and industry-specific terms. You must do it manually.
Naturally, the first source is your site search logs. Identify searches with "0 results found" and see what existing search terms they can map to, and adjust your search tool's vocabulary to accommodate them.
But this only helps for searches that have already occurred. You want to also be proactive for your most important products. So, in addition to site search logs, you can use any of these 6 additional resources:
1. Keyword research tools like Google Keyword Tool, Wordstream, Wordtracker or Keyword Discovery.
2. Customer reviews (both yours and "theirs"), which helps you understand what product attributes are important to customers and may identify long-tail opportunities
3. Competitors' descriptions and title tags - they may know something you don't know based on their own research and data.
4. SpyFu, a paid search competitive intelligence tool shows you what your competitors are bidding on in PPC campaigns. You can take out a one day membership for just a few dollars.
5. Use the Google Analytics hack that shows you the long tail terms Google is matching your ads to. You might find some gems in your paid keyword referrals. Update: Google Adwords has changed its Analytics reporting, you no longer need this hack. Find this data with Traffic Sources>Adwords>Keywords>Matched Search Query> and refine by Match Type or Keyword.
6. Brainstorm. Sometimes you can come up with misspelling ideas that don't show up in the above tools. Ask coworkers how they would spell certain products. Try doubling letters like "L" and "R" or singling them when they are actually doubled. Add apostrophes where there shouldn't be, and remove them where they should. Break apart compound words like "antivirus" into "anti-virus" or "anti virus." Meld words that should be separate like "leg warmers" to "legwarmers." Get creative.
True, this can be a very time consuming project, but it's worth it for certain terms. Figure out a way to prioritize which terms need optimization first, and work down from there - e.g. start with flagship products, top sellers, top seasonal products, highest margin or top misspelled terms.