Have a nice pair of scissors for fabric cutting and one for paper cutting; you may even want one for thread cutting while sewing. I use Gingher 8" Spring-loaded Dressmaker shears. They are fantastic and run about $50 a pair. However, you can find them for as little as $25 on eBay. Another trick is to keep them sharp with scissor sharpeners which you can find at craft stores.
2. Iron, iron, iron!
Have an iron ready and one that is preferably powerful and can press steam through quickly. I bought mine for around $30. It works great and speeds up the ironing process. I also keep a spray bottle with water in it and a pressing cloth. Another nice thing to have on hand is a sleeve ironing board. It can also double as a tailor's ham and facilitates ironing sleeves and darts.
Finish all seams with a serger. You don't need a fancy one just a three thread will do. This will make sure that the fabric will not fray.
4. No stress hems and sleeves.
Have a sewing machine where you can take off its arm for sleeves and hems.
5. Perfection is the golden rule. If it is not something you are proud of then don't wear it.
6. Use newspaper print paper when making patterns.
The "Golden Book"
7. Have a sewing book on hand so you can refer to it often. I also use burdastyle.com
Make sure you are using the right needle for your machine and project. Also change your needle often, as they dull very quickly.
9. Clean machine
Keep your machines clean and make sure that they are turned off and unlugged after each use.
10. Don't stress the small stuff and throw your machine out the window. Make sure the needle is correct, the threading has been done properly and the tension has been set correctly.