A few weeks ago I got a letter in the mail from United saying that my miles would expire within a few months. The mailer had an insert with a way to cash in some of my miles for magazine subscriptions. I didn't want any magazines and I didn't want my miles to go to waste either. I went to the United airlines site and looked through the options on redeeming miles but they didn't have anything I really wanted to use my miles for. I found and used two ways to keep my United miles from expiring and both were fairly easy and low cost.
Trick #1 ) Get a restaurant reservation. In the United Miles Plus shopping* system you can reserve a table at a restaurant and get 60 miles for doing so. They use the OpenTable reservation system to do the reservations. Of course I wouldn't go out to a fancy dinner only to keep my miles from expiring. It just happened that I had plans to have dinner with a friend at a restaurant that was in the reservation system. I would have eaten there anyway so making the reservation didn't cost me anything nor cause me to go spend money I wouldn't have.
Trick #2 ) Buy something for $1. There are many shops on the United shopping site. Some of the shops have free shipping and cheap items. I arbitrarily chose Newegg.com since I'm familiar with them and I buy computer stuff there on occasion. I searched Newegg and I was able to find a roll of electric tape to buy with free shipping that was just 99¢. Buying that item for 99¢ got me 1 mile which extended the expiration of my miles. I don't really need electric tape so I'm kind of wasting a dollar on it but one dollar is worth spending to keep from losing 11,000 miles. I don't see electric tape there anymore but someone is selling many Magic the Gathering cards for 99¢ with free shipping. I was initially worried that only spending 99¢ wouldn't get me a mile, but they rounded it up and I got credit for one mile.
One thing to watch out for is that the miles earned through the shopping site might take several weeks to hit your account. You need to do this well in advance of the expiration date for your miles to make sure it works. The United website says that it can take several weeks for the miles to be credited to your account. For me it took around 2-3 weeks to see the credits.
It would be best to plan to use these tricks in advance. If you dine out at a restaurant in the OpenTable system with any regularity then simply remembering to use the United site to make your reservation 1-2 times a year would keep your miles from expiring. Or if you do online shopping at any of the stores in the United shopping site then you can use their site to start your online shopping and get miles that way. This way you can incorporate some free United miles in your normal activities and not cause any unnecessary or unplanned spending.
* Unfortunately as I write this today and yesterday I have not been able to get to United Plus shopping website. It looks like the server is down. I assume that is just a temporary problem and hopefully its fixed by the time you read this.