you do it very carefully, as it is very easy to violate a corporation's trademark without trying.
first step; make sure you know your domain is generic. If you own HostingSolutions.com, then you are probably fine. If you are trying to sell eBaySolutions.com to Ebay, then watch the hell out.
second step; make sure your letter is highly professional and courteous. make it clear that you are not spamming them. give them some great reasons why purchasing your domain would be good for their products, or their branding efforts, etc.
Don't press them too hard to buy the name, if you put pressure they are far less likely to want the name.