Combine visuals with text in a balanced and useful way. If users have to find a product to solve a problem, then have images of the products along with the names and provide a filter to quickly "shorten" the list to zero in on the right product.
Offer "Frequently asked questions" for the product first. Use analytics and the data from your customer support system to "push" the most frequently asked questions/problems so that (hopefully) 80% of the problems users have with the product are front and center. This takes the cognitive load off the user and puts it on the system (where it should be). Refresh these questions with new data and analysis often.
Use smart search. Once the user finds the product they are having a problem with, provide a search field. And make it a "smart" search, one that maps results using keywords and meta-data to your knowledge base, product manuals, etc. Push the most "likely" result to the top of the list. Again, see the "cognitive load" note above.
Don't ask patronizing questions. A question like "is the power turned on?" Is insulting and off-putting. Define a content strategy for any guided questions/processes that sets the proper tone of voice.
Provide an "easy out" after the second or third step in the process. Either provide a text field that they can submit the issue to support or a phone number or both. Let them self-service at first but let them "pull the rip cord" if they don't quickly find the answers they want.
Give users a tracking number (and e-mail it to them as well). Let them know your company WILL follow up and give them a way to track the support request with a unique ID. And design your service process so that every part of your support team has access to that number. This article from a favorite writer of mine shows how NOT to provide customer support: Remote Possibilities, Part 2