Start your oil paintings with an acrylic underpainting - When you start an oil painting using oils, it can take some time for that initial layer to dry. Starting your painting with an acrylic underpainting however is a wonderful time saver as acrylic paint dries quickly and oil paint can be applied over the acrylic.
Make certain you have good lighting - I didn't realize how important good lighting was until I painted outdoors for the first time. Good lighting brings out the color and also lessens the strain on your eyes. If you can, paint in a place that gets plenty of natural sunlight. If you cannot afford this luxury, purchase a good indoor light. Check out http://www.ottlite.com/ for a great selection of lighting products.
Make sure you have good ventilation - If you are using materials like oil painting thinners and cleaners in your studio; make sure you have good ventilation. Some of these chemicals can be quite toxic so use caution when working with these products. Consider using a water miscible paint like Grumbacher Max Oil Paints. Max Oils can be diluted with water thereby eliminating the need for solvents.
Fat over lean - Follow this rule and you will reduce the chance of your paint cracking. Each layer of your oil painting should have a higher oil content then the ones below it.
To create the illusion of distance in your paintings paint receding objects with cooler less intense color. Objects that advance are warmer and more intense.
If you are feeling uninspired don't get discouraged. Try taking a walk outside, breath in the air, look around at the beautiful earth God created. Try playing music while you are painting. You will be surprised how music can affect your painting. Visit a museum or local gallery. Viewing other works of art can really get your creative juices flowing.
When holding your brush avoid holding your brush like a pencil too close to the bristles. Oil brushes are made long for a reason so that you can paint further away from the canvas. Practice holding the brush toward the middle and end of the handle.
Maintain a clean organized working environment - Get yourself into the habit of keeping your work area clean and organized. Have an abundant supply of rags or paper towels nearby. Get a few glass jars for storing mediums, solvents and your used brushes while painting.
Brush Selection and Care - You should probably invest in a good set of brushes. Cheap brushes are not recommended as they shed their bristles quite easily while painting. I prefer working with Hog Hair brushes, but oil painters also use sable and synthetic sable. Avoid nylon brushes, as these are better suited for acrylic paint. My brushes include a variety of flats sizes #3, #6, #8, #12, a #4 fan, and a few small rounds for detail work. Selections vary from artist to artist, depending on painting style usually, but the above mentioned work fine for me. Take excellent care of your brushes. This is very important, especially if you have an expensive set of brushes. You may want to do a search online for more in depth brush care instructions. If you do not clean and store your brushes properly, you will ruin them, simple as that. Use a quality brush cleaner and preserver that you can purchase online or at your local art store.
I hope these oil painting tips have helped. Oil paint is not the easiest medium to work with. I recall the first panting I ever completed. It was a complete nightmare, and I almost threw in the towel. I have been painting for over 5 years now and I am still learning and improving every time I pick up the brush. Don't give up. With time and practice it will become easier and more rewarding.