How to Detect and Avoid Performance Loss?

I just got a 13" MBA with the i5 processor and Lion (the 2011 model). I have about 100 GB of audio/video, lots of RAW photos, Win 7 running as both BootCamp/VMWare, MS Office Mac, and lots of Adobe products.

My Question: I'm accustomed to Windows getting slower and slower, until it crawls to a halt. Will this happen with my Mac? Are there things I can do to avoid or minimize this? Are there good ways to monitor performance in order to detect a slowdown?

I've used Macs, Unix and Linux systems before and I'm comfortable with using Bash and stuff. Thanks in advance!

 

Time doesn't slow performance, software does. And that is my question.

 

My daughter's Mac Mini was performing very badly: It would frequently pause for 20 seconds during ordinary tasks. First, I performed all of the usual tasks:

• Booted from the recovery partition and ran Disk Utility
• Updated the software
• Disconnected 3rd party USB hardware
• Checked for sufficient disk space

After the system update, the problem seemed to be gone, but I'd already made an appointment at the Apple Store, so I brought it in. They ran a diagnostic which reported no problems, so she took a look:
First, she moved the 1,000 photos which my daughter had on the Desktop. Yes, 1,000 icons. What can I say, she's twelve?
Second, she deleted the caches, located in /users/username/Caches. The caches were about 1.4 GB in size. It took a long time to empty the trash. I didn't know about the Caches folder. So far, so good.