My first fortress made it all the way to becoming a mountainhome, but not without some headaches. Here are some lessons I learned the hard fun way:
Don't make the fortress guard (the one led by the captain of the guard) into an elite military squad. In fact, make the dwarfs in charge of beating up vandals as weak as possible.
If you decide to dump water into a cavern rather than build a proper pump loop, make sure you pick a cavern you don't want to use ever again. Or maybe just bite the bullet and figure out how to make a proper pump stack.
Always put three or more doors between the fortress and anything that you might want to lock off in the future. You never know when a couple of lettuce seeds are going to keep that door from closing.
Don't let dwarfs into areas that have even slowly flowing water unless you're okay with them going the same place the water is going.
Don't put your rooms one z-level below trees that you might want to chop down. Unless you like unplanned skylights.
Don't put a hatch next to a pasture.
Locking your tantrum-prone kids into a "Grumpy Dwarf Suite" (a fancy bedroom/dining room with a bunch of toys and kittens) is surprisingly effective, but...
Make sure you put some cups and clothes in there too.
Put it somewhere that dwarves aren't going to travel to accidentally. They may start stealing the food and drink in there faster than you can restock it.
If you have a strange-mood dwarf who has claimed a workshop, under no circumstances should you forbid the blocks that the workshop is made of. Like not even for a second. The bastard will go mad instantly.
And there are also some lessons I learned in the hard (but not fun) way. Like, they ended up causing me to waste a lot of time doing repetitive tasks or ended up slowing down the FPS.
When causing intentional cave-ins, be 100% sure that there are no floors, walls, or ceilings connected to the part you want to drop. And then check again. And check again. Resetting up the supports and switches is a pain in the ass.
Only cultivate one or two livestock species for meat/wool/milk, and only keep maybe one or two ungelded males. Maybe put them in a different pasture when you've got enough animals. (Having too many animals might sound like "FUN", but the effect on FPS is significant, so I understand.)
Make an atom smasher immediately, so you can keep the total number of objects in your fortress lower. After a while, it will take a solid 5-10 seconds just to open up the objects menu, and you want to put that off for as long as possible.
Go easy on the conditional orders. There are some jobs, like throwing out rotten garbage, that really need to happen soon, but don't seem to have a very high default priority (and as far as I know, they can't have that priority increased). If all your dwarves are busy making crap you don't need, those jobs might never get done.
The tops and trunks of fungus "trees" count as walls and can prop up things that you want to collapse just by being next to them.
The "places" tab is your friend. You don't have to click on every single temple/guildhall just to figure out which deities/guilds you've already got space devoted to.
There are also some lessons I haven't learned yet.
How to make a marksdwarf squad that will actually equip bolts more than once. (I've tried all the recommendations out there, and while some worked to get them to equip bolts once, the dwarfs went back to no bolts almost immediately.)
How to keep my religious dwarves happy. (I made so many temples to so many gods, but a large number of dwarves just refused to use them no matter what.)
(I'm also trying to decide what to do with my mountainhome now. Doing anything interesting with it takes forever at this point. I could just retire it. Or I could go visit the circus. Or I could declare war on the elves. Open to suggestions.)