As I work to make weight training a part of my fitness routine, how do I make sure I'm not overtraining? Are there some general principles to follow, or signs my body might give me, that I am overtraining? I certainly don't think I'm overtraining right now, but I still think it's something for me to be cautious of. One could, I suppose, recommend some well-defined routine that would be effective at building strength at a reasonable rate without overtraining, but since everybody's body is different, I was wondering if there are some ways of knowing that I am overtraining and need to take it easier or, on the other hand, not overtraining even when I might suspect I am and, in that case, I should feel free to maintain if not increase how frequently I'm training.

Overtraining is a mythical beast that is often spoken of but rarely ever seen. Eat and sleep well and I doubt it'll ever be an issue for concern

However, I understand that pretty major depression is a possible sign

it is possible to overdo it, but as CCR said, it is seldom seen, most people would lose motivation and thus stop training before overtraining actually take place. Just get a well structured routine, eat enough and sleep and you're golden!

Major depression could be a sign that you're following the same routine over and over again. Depression and stress are the end results of allowing something to build up over time; the best thing to do is realize a potential problem before it is able to build up to a point where it's a depression or stress factor. With going to the gym, simply allowing rest and mixing up the program from time to time (ie. going outside to run or something) can prevent you from becoming depressed or stressed.

For overtraining, it's varies from person to person. No one can tell you that you're overtraining, and there isn't one factor that determines overtraining. Physically pushing your body can be both exciting, but exhausting. The only easy day was yesterday. Just listen to your body and understand your limitations. Now if you're like one of those people who walked 1.5 mph on the treadmill and then call it a day cause your legs are sore, then you need to check it. But if you just ran 5 miles, and lifted some weights, it might be a good idea to call it a day.

there is no such thing as overtraining just undereating
TBH overtraining is an overused word but can happen some of the signs are You are experiencing over use pain, and strain symptoms in joints and/or muscles.
You are tired and irritable all the time, yet don’t sleep well.
Strength levels are significantly decreasing,
dont train to faliure on a regular basis,eat well,sleep well and there shouldnt be a problem.

^^ What he said.

most overtraining refers to athletes in a high school or collegiate setting. Some of these kids are training year-round for multiple sports (competition, practice, conditioning, weight room). Most who experience this are not giving themselve opitmal time to rest or allow proper progression as their training cycles change. It's an unfortunate thing I've seen happen (and well-documented). Most fitness enhusiasts or recreational athletes worry about this but they are coming nowhere close to the type of volume some of these kids are taking on. For the general fitness population, I say don't even think about overtraining. Just worry about plateauing