1. You need to take care of edges. Smooth them and where is sand on top build floor and ramps. Trees are your enemies here. When ramp going down around trees, you need to build floor 3-6 tiles from the edge. Trees block access, but animal still spawn on edges. Make sure every side have ramps, so you get access to every of 4 edges. For your hunters, wandering animals, if not for merchant's wagons.
2. Always smooth every level of wall after channeling down. Be paranoid. Seriously. Your worst problem could prove channeling suddenly into aquifer, which is a huge issue when digging into mountain side. In sand you have to make obsidian.
3. When you lower terrain to lowest level (usually level of the river in area), don't forget you will go missing soil for under ground and above ground farming. Collapse by cave in first lower layer, so you get under ground soil. Build walls on top of it. Cave in top most soil level for above ground level farming and pastures on top of those walls. You can deconstruct walls after cave in and enjoy under ground farm plots in your fortress. Never ever cave in upon dug out tunnels and rooms. Constructed floors won't survive a cave in. Ensure always the layer is solid wall, before caving anything above.
4. You can leave walls as smoothed rocks, but inside of fortress you need to channel in for above ground-light and then roof it from outside.
5. Water. Without river, aquifer or under ground river, you are doomed to either muddy dark pools or equally muddy cavern cisterns. Make sure to pump water first. Don't use wells directly on those. Make sure your fortress cistern is rather deep and tiny, then shallow and wide. Build floors on bottom and all walls on sides. If you get lucky you may get away without any mud on bottom. No water means no obsidian from magma and you can't buy water these days from merchants. Even buying stones and logs is problematic.
6. Ensure your fortress is dried and mud-free after building _underground_ 3 z-levels mass pit. Don't build it on surface. Else you will have fun with suddenly free goblins and trolls, while trying to pit them. LOLZ!
7. Ensure your entrance is on 1st under ground level, so your Dwarves will clean entrance from any squabbles with wild live and see that bridge is under roof, so you get easier to clean entrance from eternal blood pool created by rain. Though cleaning in next DF 0.44.06 will be improved by Toady immensely, so I won't even mention issue with spilled alcohol then.