The ideal time and way to count the omer is to wait until dark, stand up, say the blessing, and then count the omer for that day.
SITUATION You want to count the omer before dark (as is often done in synagogues at the end of ma'ariv).
STATUSSince the correct practice is to count the omer after dark, you should:

• Count again with a blessing sometime after dark, or else 
• Count during the following day (but before sunset) without a blessing.

WHAT TO DO  Do the counting without saying the blessing and make a condition (mentally or verbally, either is OK) that if you remember to count the omer again after dark, the first time you counted, does not count (!) Then, if you do remember after dark, say the blessing and count the omer for that day a second time.