Dangling dims aren't always possible to avoid. They are lost when you remove a surface instead of moving it. A few things to help you keep your dims. If you go point to point it is a bit easier to loose the correct dimension for example if you have a box and dimension point to point then you decide to add fillets into the model. Now your dimensions may have become dangling. If you select the edge instead of point to point your dimension may now show from arc start to arc start. If you select the two ends "Edge to Edge" and then add the fillet your dimension should stay put.
If you have a bar and dimension it's length edge to edge and then add a cut feature making the bar shorter you now will lose the association of the length dimension because there is a new edge.
If you do the same but dimension using the long edge instead of edge to edge your dimension will stay because the edge hasn't changed.
If you shorten the bar by editing the features that are there without adding features you should never lose the association because you aren't removing surfaces and adding new ones you are just moving them.
In short avoid removing edges, surfaces and points when changing a part. This will also help with assembly mates breaking because the entity used to create the mate is gone. At times it is imposible to avoid this. In these cases try to reattach the dimensions if possible if not you are stuck recreating them. As I understand you can drag the ends of drawing dims to new features in SW 2005 much more consistently than in previous releases. Another thing you can do is Insert Model Items. These dimensions will stay a bit more consistently but they come with their own draw backs. I don't use them often.