One of the common mistakes home owners make, when painting interior of a house without hiring painters, is either paying too much for the paint or too little, and not spending enough money on quality tools(brushes, roller covers etc).

Price of paint.
When choosing the paint for the interior there is no need to spend more than $20 per gallon. Usually Behr(Home Depot paint) falls into $20 category. Glidden $17(Home Depot) another paint that can work just fine for interior painting. When you go to a retail store to buy paint and not sure exactly what brand, type and price range you are looking for, and ask store employee to help you with the choice, most often you will be shown paints that are on expensive side. That is how store employees are trained, and they are just doing their job of selling you the most expensive paint. But even above mentioned (Glidden and Behr) inexpensive paints on interior walls will last almost indefinitely.
However, if you buy the paint that is too cheep, it may not cover well and you will have to apply 3-4 coats of paint where it normally would require only two coats. Besides coverage it may also not look as good as a better quality paint would, when applied on the walls. And it would be not as easy to clean as decent quality paints. A 5 gallon bucket of maintenance paint from Wal mart, with a price of $40 would be a good example of the paint to stay away from.

Painting brush.
When choosing the brush, no need try to save money here. Cheep brushes are very hard to work with. They don't hold enough paint and always impossible to paint a straight line with, even if you are a pro. Quality brush price tag is about $17 for 3 inch brush. “Purdy” is the brand name synonymous with quality brushes.

Roller cover.
Quality roller cover is never sold in packs of 3 or 6. It always comes as a single item at a price of around $10. The best are “sheep skin” or “ lamb skin”. They come in different thickness and the choice will depend on the roughness of the surface you need to paint. Cheep roller covers don't hold enough paint to cover even one line from the top of the ceiling, to the floor of a room 8ft in height. With quality roller cover, you can paint 3-4 lines before you have to dip your roller in paint again.

Painting process.
Another mistake is painting with a roller as a first step of a painting job. First, all painting needs to be done with a brush.
1 Lines between the ceiling and the wall.
2 Around the outlets and window jams.
3 Around the trim of the doors, baseboards and corners of the room.
4 And the last step is to apply paint with a roller on the areas in between.
This sequence will help to avoid lines between the areas painted with a brush and areas painted with a roller.

Paint sheen.
Buying the paint that is too glossy. If it is your first time painting project, flat paint is the best choice, as it will leave little or no brush marks. Semi-gloss or full gloss requires maintaining a constant “wet edge”. Those two sometimes can be tricky to apply even for a professional painter.

Cutting a straight line.
Trying to paint a straight line between the ceiling and the walls using tape. While doing so on the smooth surface may come out ok, most of the drywall in Arizona is textured. No matter how hard you press the tape against the ceiling, the paint will “bleed” through. That is why all the lines between the ceiling and the walls are needed to be done by hand without using any tape.

Second coat.
Being disappointed after first coat did not come out perfect and thinking something went wrong. First coats are almost never perfect and 95% of all painting jobs will require second coat.