I could make up some long winded post about perspective and proportions etc, but it would just be me talking out my blowhole. Keep in mind, this is MY first SW tank ever, so in all likelihood, I got lucky. =P
The best pointer I could give is to play with your rock. I had my dry rock for probably a month before I got it wet. Pick up the pieces, turn 'em over, flip 'em around, get an idea of each rocks personality. Got an ugly rock? Try breaking it, worst case, you end up with some rubble to spread around the base of your rockface which makes it look a little more natural. Got a solid hunk of rock thats just plain boring? Use masonry drill bits and think like a mantis shrimp (minus all the carnage and bloodlust
). As for caves, think of them more like tables. Use smaller less attractive pieces to support large flat pieces, or there's always PVC scaffolding. Then take some of the nice looking pieces and place around the sides to form walls, just make sure there's plenty of water flow through the cave and more than one entrance so fish can't get trapped inside by a bully.
For arches... i got lucky, I dropped a nice large rock by mistake and it broke into an almost perfect arch. If i hadn't gotten so lucky, my plan was to use alot of the smaller rocks and rubble I had and some of the aquarium safe epoxies to make a single arching rock. The few times I've done things like for freshwater the trick was to think like an artist first and an engineer second. Sculpt it how you want it, then use some epoxy to make it stable on at least one end. I say one end because you'd generally want to anchor an arch off a large stable rock to help it fight the tendency to fall forward or backward. I wouldn't recommend epoxying both ends because that would probably make it a pain to get back out of the tank if you have to take the system down.
Just remember, an aquarium is its own little world and you are it's god. Try to work with your building blocks to make the world in your image, but if all else fails, your only human, so break stuff till it works!
/close blowhole