Find a small space -- preferably near a mirror -- where you can stretch and practice the basic positions and steps that students of ballet typically perform at a ballet barre. Clear the area of throw rugs and clutter. Use a sturdy chair, window ledge or door frame for support. Practice plies (gentle bending of the knees to warm up the legs), tendus (slow brushing of the feet along the floor to warm up the the feet and and ankles), degages (a brushing of the feet in which the toes lift slightly off the floor) and -- if space permits -- grand battements (large, powerful leg lifts to build strength and flexibility of the legs and hip joints). Work slowly and deliberately, focusing constantly on your use of proper technique.
Work on fine-tuning the positioning of your hands and feet if your space is very limited -- such as on an airplane or in the passenger seat of a car. Recall what you have learned about proper placement of the thumbs while relaxing your fingers at the joints. Remove your shoes while you are sitting, place your feet on the floor in front of you and practice rolling slowly through your feet onto pointe. Check that you are not "sickling" your toes (angling them inward) or "winging" your toes (flaring them outward) excessively.
Take notes immediately following every ballet class. Write down the names of steps that are new to you as well as your instructor's corrections, including comments she made regarding your posture or your execution of particular steps or combinations. Whenever you are practicing, refer to your notes and attempt to self-correct.
Watch short instructional ballet videos as you practice. Search online for instructional videos that feature clear demonstrations and explanations of proper technique. Build your visual memory by watching and analyzing a video clip several times before shutting it off and attempting proper execution of the step or movement combination.
Read books and articles that focus on dance anatomy and kinesiology. Scour dance literature for tips to improve your ballet technique. Familiarize yourself with different imagery and alternative ways of thinking about turns or jumps. Test the different approaches when you practice on your own.
Visualize different steps and combinations without performing them physically. Recall everything you know about proper execution, and visualize yourself performing your movements precisely and with ease.