I have been researching my family history for a very long time, both the right way and the wrong way. Genealogy the wrong way is frustrating to both yourself and anyone else who might see your research. These rules will help you to keep on the right track - they follow the philosophy, "Work smart - not hard." Feel free to add more in the comments.
These are in no particular order.
1. The most important rule of genealogy is: DOCUMENT YOUR SOURCES. This cannot be stressed enough. Anyone who looks at your research (including yourself weeks, months, or years later) should be able to reproduce your work step-by-step. By keeping track of not only those sources that give you the answers you need as well as those that don't, you won't waste time going back over the same dead-end sources time and time again.
2. GO BACK TO ALL OF YOUR SOURCES TIME AND TIME AGAIN. You might have missed something the first time. Each new piece of information you get provides clues that might make some insignificant, supposedly unrelated detail all of a sudden highly significant. It might be the one piece of the puzzle that helps you break through that brick wall.
3. NEVER "THROW AWAY" INFORMATION. Just because one document gives a different birth date or a different spouse, do not dismiss this data before you thoroughly weigh the evidence. See #4.
4. KNOW THE SOURCE OF YOUR INFORMATION. By this I mean something very different from #1. Who provided the information? For example, for the date of birth, a birth certificate completed by the parents on or shortly after the date of birth would be more reliable than a death certificate completed by the attending physician, or a pension application completed by an individual whose financial situation might be affected by his age. A church record filled out by a priest or minister would be much more reliable than a census enumeration whose source is unknown.
5. GET LOCAL. Know the geography of the area in which your ancestors lived, not just physical terrain but also political jurisdictions. Know the laws that governed your ancestors' time. Know the local history - the local leaders, the local churches, the common occupations of the area at that time. All of these help to recreate the world in which your ancestors lived, and all of your "evidence" was created.
6. USE ORIGINAL RECORDS WHEREVER AVAILABLE. There is less room for transcription mistakes or other signs of sloppy work (someone else's, of course) when you are able to go back to the original records to verify your information.
7. DON'T BE AFRAID TO USE DERIVATIVE AND/OR SECONDARY SOURCES. Is the information from a local history book or a "family tree" with no sources cited or the IGI going to be 100% correct? Absolutely not. But that doesn't mean that it is necessarily 100% incorrect either. Follow up on other people's research - if they cited their sources go back to the original - if they did not, then try to find records to either prove or disprove their theory. These references serve as great starting points for your research - they allow you to start with one fact and verify it, or toss it out. But either way, whether positive or negative, it is progress.
8. NEVER ASSUME ANYTHING. For example, not everyone listed in a pre-1880 census household is the child of the head. Younger women might be daughters-in-law rather than daughters. Older women might be spinster sisters rather than wives. Even a younger woman might be a wife rather than a daughter. Assumptions can lead to some very convoluted, and very wrong, family trees.
9. NOT EVERY PROBLEM CAN BE SOLVED. While there may be enough information to suggest the possibility of a certain fact or relationship, there may not be any direct evidence to prove it. Sometimes you may not even have that much, and have to move on to a different branch. Remember that the further backwards you move, the less likely that your ancestor created certain records, or that these records are still extant. A lot of courthouses burned, through natural disaster and war. Laws creating certain records were sometimes passed very recently. And of course that "KEY" piece of evidence was inevitably thrown away during spring cleaning.
10. REMEMBER WHY YOU DO IT. Genealogy should not be a goal in and of itself. It should not be "just" to get into a certain lineage society or even for bragging rights. Genealogy is at its core about family, and knowing those who came before. It has been said that "a tree without roots cannot grow", and in my opinion the search for roots can be a means toward growth.