Attend class faithfully; only a true emergency or crisis should prevent you from going to class.
 

Arrive to class early and stay for the ENTIRE time.
 

Pay ATTENTION to what is going on in class and participate to the extent that you feel comfortable.  Talking to your neighbor may be more fun but it is also quite detrimental to your purpose for being in class (to learn what you REALLY need to be doing)...
 

Do your HomeWork faithfully.  Attempting the HW as soon as possible after class is generally the optimal time (as things encountered in class will still be fresh in your mind).  Once again, only a real emergency or crisis should delay you from doing the HW assignment(s).
 

Figure out the instructor's grading scheme and keep close tabs on your grades, especially your exam/test scores (as they are generally a good indicator of how well you are doing).
 

Whenever you sense you are having a problem (no matter the size) in keeping up, GET HELP.  The sooner the better, and usually the instructor is your best resource despite any lack of comfort you may feel in asking him/her for their help.
 

Study for your exams 5-10 days before the exam date.  Although it is very tempting to make a heroic last minute effort to cram, if you are mortal then your best bet is to break the task of studying into periodic intervals well ahead of time (which will also relieve you of the "now or never" panic/stress disorder).
 

Get to know others in your class; if you feel weak in the subject then you should try to find someone who does very well -- rather than bond with another person who is struggling (as typically you will merely reinforce one another's mindset for rationalizing your present condition versus discovering how to improve it).
 

Join or form a study group that meets REGULARLY.
 

Make 3" x 5" (or 4" x 6") note cards with important ideas (formulas, properties, etc.) and carry them with you so that you can frequently glance at them throughout your day wherever and whenever you have a spare minute or two.