One thing that I have learned from our adventures in home renovations thus far is that you need to have a contingency plan.
This advice is two fold: first of all, you need to plan to have an extra percentage over and above your reno budget on standby in case one of the dreaded "what ifs" come to pass. No matter how well you budget, even down to the exact brad nail and ounce of grout, those sneaky little circumstances rear their ugly heads. Suddenly you are re-drywalling a section of the wall, or re-plumbing your tub. I'm not exactly sure the percentage the pros say to have as a contingency over and above your alloted budget, but my advice is to figure that out and don't be surprised when you actually have to pull the proverbial wad of cash out of your back pocket to pay the furnace guy (yes, again, a reference to the great furnace/hot water heater debaucle of '09).
The second part of that advice would be to expect delays. As you head full force into the renos, past the point of no return, keep in mind the ever present reality that it just may take longer than you hoped. Again, no matter how well you plan, right down to the grout-sealing-second, things can suddenly put your scheduled reno on hold, or set you back days (or, heaven forbid, weeks). Funny enough, these setbacks usually work in seamless cooperation with the extra money you are forced to shell out. Not "haha" funny, so much though.
What brings this advice about, you ask? The bath renos are going well, don't get me wrong. But it is costing a bit more than we had planned, and won't be done until later this week. Luckily, we planned for those "surprise" expenses, and we haven't worn out our welcome at my parents. And if they do change the locks on us, it will just be further incentive to get that bathroom done.