First up, either know what your mortar is supposed to look like or buy it premixed.For the record, it is supposed to look like creamy peanut butter. Do not trust the instructions on the bag. The first time around, I followed the instructions on my bag to the letter. The mortar was very, very dry, but it specifically said, right there in big letters, DO NOT ADD EXTRA WATER. So even though I had many misgivings, I followed directions. And I paid for that. Not only did half of my (not cheap) bag of mortar dry out because it took so long to get the tile laid because I was fighting the mortar every step of the way, but pretty much all of the tiles (read: the entire main body of the floor, every whole tile in the bathroom) popped up at some point in the next few months. In a way I guess that's good, because now I know they're really set.
This was my first tiling project, but I chose to use the dry mortar because I kept reading that the premixed wasn't "real" mortar and thus was somehow inferior. Well, real mortar or not, it holds the tiles to the floor. That's all it needs to do. If you have any misgivings or doubts at all about how fast you're going to go or about your ability to get the consistency of the dry mix mortar right, just buy the premixed.
Second, don't wait to clean the mortar from between the tiles. This is another thing the instructions on my bag said to do. "Just let it dry!" the pictures said. "You can scrape it out the next day with this pointy mortar-scrapey thingy, no problem!" LIES. I don't even know how long I spent with the Dremel grinding that crap out from between the tiles. Take a tile spacer or popsicle stick or something the width of your tile spaces and get that mortar out ASAP. It doesn't have to be perfect, but it does have to be at least half the depth of the tile.
Third, don't let it fester. Now, because of my extra-dry mortar, I have no proof that this is actually an issue, but I have strong suspicions. I started this project on New Year's Day. If all goes according to plan, it will be finished by the end of this week. Mid-April. That's over four months of water from showers (and a couple instances of the toilet overflowing) getting in through the ungrouted spaces and under the tile at the edges of the room where I hadn't laid the tiles that needed to be cut. Like I say, I don't know 100% that this caused issues, but I can't imagine it was good for anything. Supposedly the mortar is unaffected by water because it dries through a chemical process, but better safe than sorry. When a project isn't going right for me, I tend to shut down and ignore it for as long as I can stand. I have only myself to blame for any damage water did, so learn from my mistakes.
Fourth, have the right tools. When I started out I thought I could get by with a score-and-snap tile cutter and tile snips. My mom used the score-and-snap cutter when she laid tile and had no issues. However, I was laying porcelain tile. I didn't realize this when I bought it (I just bought the cheapest thing that was the least ugly) but porcelain tile is much harder than ceramic. The tools I had did absolutely nothing besides break the tile; eventually B banned me from using them anymore. I ended up cutting them with my dad's RotoZip with a diamond blade. It worked wonderfully.
I hope someone out there can learn from my mistakes, because they were so annoying I was almost put off of DIY tiling forever, and I don't want anyone else to feel that way if it can be avoided. Happy remodeling.