Internet Monk has ten good ones. Item number one on his list is not to try to harmonize the gospels. Don't worry about making the four gospels "come out right" in terms of historical factoids or chronology. For example, don't try to make the four crucifixion accounts "mesh" in terms of timing or the events described. (You couldn't even if you tried.)
Likewise, the Christmas accounts in Matthew and Luke really should not be combined. In the first place, you can't really make it work very well, and secondly, neither narrative is really about historical facticity anyway.
Similarly, one should avoid interpreting, say, Luke, by citing something in John. Luke has his own point of view. John has his. They are not the same. Don't think that by "mooshing" everything together that you're somehow getting a more complete or whole story. Tatian tried doing that back in the second century, but the church never really took to the product he produced.
Each of the four gospels has its own integrity and its own point of view. The problem with harmonizing is that such an endeavor undermines that integrity and creates, in effect, another gospel account distinct from the four we already have. It says that Matthew, Mark, Luke, and John were wrong. They have been replaced by the Gospel according to St. Cuisinart.
Don’t harmonize the Gospels. That’s like taking four paintings and combining them into one. You come up with something no one painted and no one intended to paint. Let each Gospel author be an artist in his own right. However, a Gospel synopsis, such as those available from UBS, are very useful and important in comparing Gospel texts to one another WITHOUT harmonizing them.
I second Internet Monk's mention of the synopsis as well, especially keeping in mind Markan priority, and that Matthew and Luke basically reinterpreted Mark. It's always interesting to see how Matthew, for example, subtly changes Mark here and there. In Mark, James and John want to be next to Jesus in his glory. Matthew changes this slightly so that it is James and John's mother who asks Jesus for this special favor. You have to wonder why. Questions are good things. One even wonders: Could fundamentalists spend even one hour with a synopsis and remain fundamentalists?