Try to get organized before you start census searching
Have maps or map
links handy including help for boundary changes. Wikipedia is very good
about having township maps and histories. Visually look at where a person
Check out The FamilySearch U.S. Wiki which will tell you if something happened to a state or county census as well as all kinds of other good information.
If you plan to print the census images you will save ink by learning to crop them first. Make sure you write the state, county, year etc. on the side or back as many early census do not have proper headers.
If you plan to save the images for your genealogy program make a folder to keep them in so they are all in one place.
If you are new to census searching go to Ancestry's (free) printable census forms and make a copy for each year which will tell you exactly what was being asked and what is in each column without a lot of squinting and scrolling.
If you are transcribing the census and don't want to use a form do it on a full sheet of notebook paper (as opposed to scraps) that can be filed or placed in a binder
Try to get in the habit of using a Census Inventory Form which will quickly tell you if you have found everyone in the family for all census years.
Be thorough. If you are working with Thomas and Lucy and their five kids try to follow out the entire family on all census where they might appear before jumping to another generation. Don't skip any years. Don't ignore any of the children.
Though some people got missed (particularly if they were "on the move") most census problems are caused by one of these things:
incorrect indexing / bad handwriting
they aren't where you expect them
initials or nicknames
Where to find census records
At this point Ancestry.com has the most complete census and indexing to census. They not only have every name indexes to all U.S. Census they have many state and territorial census and also have census for Canada and the British Isles if you subscribe to the World Edition
FamilySearch Pilot is free and most years now have an every name index but they are still working on 1930.
HeritageQuest is free to residents of North Central Washington. You access this site by going to the North Central Regional Library Web at http://www.ncrl.org/ Choose the tab at the top that says Research & Homework where you find a link to Heritage Quest under their Research section. You will need your library card # to sign in and if you don't have one of those you may visit the public library to get one or email at the website to have one issued and mailed to you.
Heritage Quest has ALL U.S. Federal Census but not all census is fully indexed
and some years are not indexed at all.
check the Census page here at the WVC site
don't forget the power of Google. "1860 census" "Brown County" Ohio will pull up transcribed census if it is out there somewhere on a genealogy site.
A lot has changed for census users as new records have come on line. As you find a family you can use other tools to help you along - particularly vital records or information about the people around them.
Marriage information is neccesary to good census searching because it lets you know who the daughters and sisters marry and what becomes of your widowed grandmother.
has many marriage databases
FamilySearch Pilot has made a huge push on vital records
if they lived and died in Washington check the Washington State Digital Archives Site for vital records.
If you are not finding what you want try Googling "Ohio Marriages" or "Jones County Iowa" marriages
If you don't know when they died you can't be sure if you have lost them on the census or they are not around anymore!
the databases above but also check:
For 20th century people try the Social Security Death Index (Ancestry's version is good and does not require membership)
Online Searchable Death Indexes and Records will let you know if the state you are researching in has death records online
pre-death record years try hunting for the cemetery record at
Find-a-Grave or check the
state/county pages at GenWeb
Family Tree sites like Ancestry World Tree and RootsWeb's World Connect are also very helpful - particularly for sneaking a peek at the neighbors.