Locate the two main clauses in the sentence.
Separate the two main clauses into separate sentences by inserting a period. Capitalize the first letter of the new sentence.
As an alternative, replace the comma with a semi-colon; this is like a comma, but is as powerful as a period.
As another alternative, place a coordinating conjunction after the comma. This is a connective word which equally emphasizes the two clauses. Examples include "and," "but" and "or."
As another alternative, remove the comma and replace it with a subordinate conjunction; this makes the second clause dependant on the first, almost like a continuation of the original thought.