Durable as in, not spending top dollar for let's say an XTR derailleur when an XT costs half as much and only weighs a couple grams more.

Instead of buying ultra-lightweight tire with thin sidewall rubber, I sacrifice a weight penalty and get some beefier treads.

In lieu of an expensive saddle with Titanium or Scandium seat rails, I stick with a cheaper one with chromoly rails.

Instead of a set of Mavic Crossmax SL wheels, I use some Sun Rhyno Lites with XT hubs.

etc., etc., etc.

Ask yourself this, "What do I NOT like about my bike?"

Then ask, "What will it cost to replace it?"

Then figure if it's worth it.

When I first started, and I don't really know why. I had a hang-up with cranks. I eventually bought a set of Race Face Turbine LP's in Powder Coat Red w/Black anodized rings.

Did they improve my riding? Not really enough to where I really noticed, but they made me LOVE my bike. The cost was worth every penny.

Then, I moved to stems. Instead of the stock generic one that came on my bike, I got a KORE. Again, it didn't REALLY change the way the bike felt, maybe made the steering a "little" stiffer, but not significantly. BUT, I thought my bike was the COOLEST! Again, worth every penny.

I refused to buy an expensive seatpost for years. Never could imagine spending nearly $100 for a seatpost. Well, after going through 3 or 4 $25 to $40 ones, and having some extra money, I broke down and bought a THOMSON Elite seatpost. Now, I can't imagine NOT owning one. They last forever!

If you've got the cash, then upgrade at will. However, if money is tight, then just replace stuff as it breaks.