Ticket to Ride Strategies
Finish your routes, prevent others from completing theirs

Ticket to Ride doesn't strike me as a strategy-heavy game.  It's all about making good tactical decisions - completing your tickets in light of random card draws and other players' actions.  Remember that unfinished tickets count for negative points at the end of the game.  My basic gameplay goes like this:


Choose tickets I think I can finish.  The big routes are much harder to complete in a 4- or 5-player game.
If possible, avoid telegraphing where my routes are going.  Others can block you easily if they know you need a certain leg.  That leads directly into...
Claim the critical legs first.  If I need a one-train leg, get it as soon as I have the card.
If you can block, do it, but make it a good block.  Spending your turn solely to block probably only scores you a few points.  If that player then goes around your block, he scores more points and he still completes his route.  Not worth it.
Once my routes are done, decide if a new route(s) is worth the risk.  There's a chance you could pull all 20's, basically guaranteeing you won't win.  With fewer trains remaining, I think the better play is to go for long links of opportunity.  A six-train link is 15 points, even if it doesn't connect to anything else.