Invalid or Stale Blocks

[bosshyip]

I use Slush's pool, so this question may belong in that thread.  However, if it is common to all mining programs, or if it is on the client side (my end) then I suppose it deserves its own thread.

I have a general idea of what invalid blocks are and what stale blocks are, though someone may care to elaborate.  In the past I have received a small number of these messages over time, but it has now increased to a few percent, which is more that I pay in donation and is a significant portion of my mining.

First and foremost, am I actually losing something?  In other words, if my hash arrived at Slush's server quicker (internet or load problem on either client or server side) would that have resulted in an accepted hash, or was the has doomed from the millisecond it left my machine?

If so, is there anything I should do on my end?  Prevent internet hogging applications? Slow down the overclock or tweak other program settings?