One of the most important aspects of design for 3D printing is how the model is oriented. It can mean the difference between a failure and a successful print.
Keep the following things in mind when designing/checking your model:
Models will print better with a flat surface at the base. The larger the better.
Avoid overhanging parts as much as possible. You can't print in mid-air! Keep in mind that printing at a 45° angle will still work.
If steep overhangs are unavoidable, use a preset that generates supports (e.g. the 'Detailed' preset). It is best practice to reorient the model to reduce the amount of support material needed, reducing the amount of filament required to print your model.
When a model is selected, a rotation tool will appear for each axis. Click and drag on the arrow icons to rotate your model, using the compass as a guide. Similar to the other modifiers, you can type in a custom rotation, accurate down to 0.01 degree.