1     Using Cheap Paint
If you think you are going to save money buying a cheap paint, remember, cheap is not bargain. You’ll need more of it to cover the same area. And since it won’t cover well, you’ll have to spend more time to apply extra coats. And after all this, it may not last as long as better quality paint.
2     Buying  Cheap Brushes
I know, you hate cleaning brushes, so you figure you’ll just buy a brushes and throw them away. Well, guess what? You have just bought expensive paint and you are going to get a poor finish because you are too cheap to buy decent brushes. You may as well have saved your money and bought cheap paint. You don’t have to buy the most expensive brushes but better brushes will give a better finish and be easier on your hands and save you energy and be faster to use.
There are different kinds of brushes. A brush that says it is for finishing will give you a better finish.
3     Wiping Too Much Paint from the Brush
The common practice of wiping the brush on the lip of the paint container after dipping it is a bad one. You end up scraping off too much paint. Instead, dip the bristles an inch or two into the paint. Then slap the brush on the side of the paint container to remove excess paint. The brush will be loaded with just the right amount of paint.
4     Buying  Cheap Roller Covers
I know. You hate cleaning rollers, so you figure you’ll just buy a cheap roller cover and throw it away. Well, guess what? If your time is worth anything, a cheap roller cover is the most expensive tool you can buy. Cheap roller covers don’t hold enough paint, shed fibers on your walls, and in general are a pain to use. Any top-quality roller cover will work fine.
5     Using a Low Nap Roller Cover on Walls
A 1/4-in.-nap roller cover is good for applying a smooth finish, but the thin nap doesn’t hold much paint. A 3/8- or even 1/2-in.-nap roller cover is best. These thicker roller covers hold more, so it’s quicker and easier to apply an even coat of paint. Only buy a 1/4-in.-nap cover if you don’t want any stipple on the surface.
6     Buying Cheap Masking Tape
Don’t scrimp on masking tape. More expensive tape is easier to put on, easier to remove and better at sealing out the paint. There are many specialized types of tape to choose from to suit your task. Some can be left on longer. Some are meant for outdoor use. And others have less aggressive adhesive so you can mask off delicate surfaces. Choose the most appropriate one, but don’t buy the cheap stuff.
7     Using the Paint From Different Batches
Don’t make the mistake of thinking that you can run to the store and grab another gallon and it’ll be the same exact color. It might be perfect, but there’s also a chance it’ll be slightly different. If you have to buy more paint, plan to cover an entire wall with the new batch rather than starting where you left off. Slight variations in color or sheen on adjacent walls aren’t noticeable
8     Not Hiring a Pro
This idea is not exactly DIY, but it may be good advice for some people. Hire a good painter. If you don’t have the patience to do a good prep job, or you do not have the skill do do a good job cutting in along the edges, then it makes sense to at least get an estimate for your paint job. It may not cost as much as you think to have it professionally done and you won’t have spills on your good flooring. A poorly done DIY job may end up costing you more in the long run.
9     Not Buying Enough Paint
We’ve all done it. The can says a gallon covers 400 sq. ft., and our walls add up to about 750 sq. ft. It seems like 2 gallons should be enough. But in reality, we end up needing two coats, and the paint only covered about 300 sq. ft. on the first coat. So back to the store we go. Remember, the first coat usually requires more, especially over bare drywall. And don’t forget to add what you’ll need for the second coat. Having some left over for touch-up or to repaint a wall later if it gets damaged is not a bad idea.
10     Using Plastic Drop Cloths
Plastic doesn’t make a very good drop cloth underfoot. It doesn’t absorb the paint, so any little drip or spill is more likely to get tracked around. Plus, plastic is slippery, increasing the chance of a fall. Four-foot-wide canvas “runners” are our favorite drop cloths.
11     Letting Friends Help
 Good paint is expensive. No two people do painting the same way and friends will not be as particular as you and so you will be able to see the difference in the finished job. You will end up having to put on a second or third coat.