I would recommend that anyone who wants to mark a claim does the following (which is sort of what has been said above)
- find a place
- /setwarp USERNAME.claim
- find that warp point on the next map (unless you already know where it is)
- Find the surrounding warp points and look at the extend of their construction.
- If anyone looks like they might be using an area that you want to use, send them a PM or email.
- If all is okay, take a screenshot of the map, marking the extent of your claim and any surrounding warp points/landmarks for reference.
- Start a thread about your claim, include the screenshot.
- Inform the upkeeper of this thread (presently Obsi) about your thread so she can add it to the list.
That way, it is up to you to check for conflicting claims but you also will have a map image showing your claim if someone wants to build within it. This thread can later be updated with images and descriptions of the construction you have done.
Reading other's claim threads is recommended, but identifying the warp points on the map is a simple start that tells you who to talk to. If there is a clash, you can always remove the claim warp and keep hunting.
Some people also use torches to help mark areas within their claim, even if they don't claim on building above ground there. Not everything is visible on the map and some of the claim may be for mining purposes.