The Magic VII.
Just a tip how to develop your middlegame strength!
There is 3 different level in this theme:
1) This level is between 2000 and 2300. At this level, it's important to find a good plan for yourself. You can find a lot of example in general books.
2) There is 2 big differences compared to level 1. The first one is that here you should also look at the plans of your opponent (and, of course, try to do something against it!). Secondly you should look deeper into your plans, try to figure out how to make it work. This level is between 2300 and 2500. I am at this level.
3) This is the grandmaster level, above 2500. When the GMs are looking at the position, first they try to evaluate the position, starting with the position of the pieces. I mean who have the better king position, better pieces, etc.
Summing up, if you are at Level 1, try to find a good plan rather than sacrificing pieces and similars. If you are at Level 2, you should not look at only your plans ( a lot of people fall into this mistake, even me), but also at the plans of your opponent. I can't say a lot about Level 3, simply because I don't know much about it, but I am sure these guys are know what to do.