I have a GT Zum 8 speed. It has a Shimano HG-31, 8-speed cassette. I like the bike and have about 300mi on it. I took it to my LBS to get it tuned up and noted that when peddling backward to get the peddles in position at a stop, the chain sometimes jumps off the rear sprocket. The owner made an adjustment and I no longer have that problem. Now the chain will jump off the front sprocket when I am in first. I took the bike back in yesterday and got an explanation of what cross chaining is and how a single front sprocket 8 speed may be prone to it. He said he could install some sort of guide, but it would cause the chain to rub and make noise. This seems to mostly happen when i am down shifting coming up to a traffic light or when I am in 1st spinning around the neighborhood warming up for a ride. It doesn't really happen when I am going slow.

Is this an issue of my technique shifting into 1st when it is not appropriate or something mechanically wrong with the shifting mechanism? I am not a luddite, but am only familiar with the concepts of the mechanics of a bicycle. Thanks in advance for any help or advice.