What's interesting about mastering Level 20 is that the enemies continue to get stronger, even if you don't, so you're constantly having to find news way of re-inventing your character to remain competitive with your enemies. My personal strategy is to max out my runes and armors and make sure I have the proper skills. This means knowing what lies ahead, and the Guild Wars Wiki is my best friend for doing that.
What gets more interesting is that, since you can reach level 20 fairly early on, and you can beat the game, after you've done both these things, you need to find something new to do to keep you interested. Some of my suggestions are below.
This is a castle I found in the Nightfall Campaign. It's actually in the middle of the mission, and most of the mission has you sneaking around it. But you can come back after the mission, which is what I did, and raid and pillage it. Which I also did. Then I posted my Ranger Heros on the wall and took a nap in the shade. It was quite a bit of fun (even after I killed the army guarding it) to run around inside a castle I had conquered with the help of my heroes and henchmen (one of whom kept yelling something about throwing iron skillets).
This is a posed picture of the Guild Hall, with my darling wife and her level 20 character standing next to me. This is shortly after our three Scrimmage Matches. The First was my Level 18 W/R against her Level 19 W/R, she had her pet and pwned me badly. The second was my Level 18 W/R with a differant skillset against her Level 19 W/R and her pet. I killed them both, but nearly died doing so. The third was my Level 20 Ranger against her Level 20 Warrior. I pwned mercilessly with a Condition Spammer build that I've carefully cultivated for PVP use.
This is a larger shot of the Guild Hall, it's actually much larger then that as there are hills mountains and a dock to explore. It's almost fully outfitted to - which is rather ironic, since there are only two people in the Guild (and you can see them both in this screenshot, too).
