Things I have learned remodeling an old house...

1) Never assume.
Never assume anything is reusable, standard, square or fixable in an old house. During my upstairs remodel I thought I would be cost conscious and a green builder and reuse some of the lumber. What I found out is that the old 2x4s were not the same size as the new 2x4s which was not a shock to me, what shocked me is that the old 2x4s were not the same size as the other old 2x4s. I had some that were thicker, some that were thinner and some that were in between. That really messes up your walls when you try to put in door casings and get your sheetrock flat.
2) Always plan for the worse.
When I was redoing my plumbing, I planned that I would be dealing with cast iron drain pipes so I got all the pieces to be able to work with that material. Unknown to me someone had already done some plumbing so I had lots of PVC to work with. Bonus! That was what I thought until I get to the edge of the house and saw where the PVC and the cast piping came together, in the middle of the foundation. Planning for the worse, I was able to work around it but if I had not planned for it I would have been in a world of hurt.
3) Nothing is as easy as it seems.
We decided to tear all the lathe and plaster off the walls and replace with sheetrock, easy right? Tear down the old lathe and plaster, and put up sheetrock, simple. Except when the lathe pieces are actually what is holding up a wall that is not secured to the ceiling or the other walls, now it is replacing the wall itself. A half day job turned into a week long nightmare.
4) Don't underestimate how much trash you will generate.
Who would have thought my house held 3 TONS of lathe and plaster. If my house was on "The Biggest Loser" it would be lapping the field.
5) Refinishing wood floors is cheaper than carpet.
Well this one is probably true, unless you figure up how much your labor is worth. Me, I work for beans and cornbread so I am pretty cheap, but after 40 hours of sanding, sanding, finishing, and more sanding, I am thinking carpet looks pretty good.
6) You never know what you'll find.
I was really hoping to find a bag full of money while remodeling, or jewels, yeah jewels would have been nice. But nope, no jewlels, no money. I found about a thousand old razorblades, a mouse and a bird skeleton (not in the same place, they did not shack up together), an old medicine bottle with the top broke off, an old newspaper and lots of mud dobber nest. Still searching for that bag of money though.
7) If it can fall, it will.
Never trust something to stand on its own. If it has the ability to fall, it will. Trust me, my couch shows the evidence of it.
8) Have fund.
No that is not a typo. Have fund, lots of funds! It will probably run you 10 to 20% above what you have estimated. It is kind of like the extra nuts and bolts you end up with when you are assembling something from a box of parts. Except when you are remodeling you end up short.
9) Have fun.
If you don't have fun doing it, well, you won't, you know, do it.
10) Enjoy it when you get done. No one else is going to enjoy it as much as you are going to, it was your sweat and blood that put it all together in the first place.