Hi there... !
Discipline is very hard to do consistently. At the beginning it is difficult because you are trying to change your own behavior along with the behavior of your children. Any inconsistencies are noticed and taken advantage of by your very observant children. Therefore the first few weeks of implementing a new program is when most parents give up. It is also the most important time to stick with the program, knowing that the children are testing your boundaries and that eventually it will get easier as both you and they develop new habits.
The best book I have found for this (it is a very thin one...! ) is by Dr. Phelan called 1, 2, 3... Magic. He also has some videos if you prefer that. The book is probably available for free at your local library but you can buy it if you'd rather.
This method uses time out, as well as other tools. The more important thing is the implementation... rather than the specific method. Many parents say they tried "time out" and it didn't work. Very, very often they are not using it "correctly" or consistently so it can not work.
If you have trouble with the above suggestions you may want to consult a local professional to see if your implementation could be adjusted or find out if perhaps you have a very challenging child with special needs... with might require additional tools.
The TV program Supernanny is sometimes interesting in showing ways of dealing with children's discipline problems... (but it is sometimes overly dramatic).
Hope that helps... please hit accept and provide feedback if this answer was acceptable. If you have additional follow up questions, let me know.