What type of flies are you using, there can be quite a few reasons for what is happening. Dry flies should be used with a tapered leader, I get a lot more tangles while using a dry, weighted flies will sometimes put knots in your line, which create weak points.
Most of this is in your casting, and can't be avoided no matter what the line/knot, it generally happens on the back cast.
Esoteric, How would you rate yourself on fly casting? 1-10? Not making judgments, just wanting to see how likely it could be in your cast, I am mastering different roll casts and I would only rate my normal back/forward cast as a 6.5, just because I am not consistent with every cast especially when going for distance.
There are a few determining factors, and I would not recommend putting anything that could add weight anywhere between your fly line and your fly unless you feel you rate yourself 8-10 on forward/back casting.