Definitely better to memorize the rules for formation. There are about 4 basic rules for forming adverbs (exception of irregular adverbs), which are much simpler than memorizing hundreds of adverbs separately.
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae
Adverbs aren't too much trouble to figure out. Most of them are formed regularly, and those that aren't are a small batch that you'll quickly recognize. I recommend reading the couple pages in Allen & Greenough for an outline: http://books.google.com/books?id=Q7cAAA ... #PPA126,M1
Averbia tam difficilia non sunt ut intelligi non possint. Praesertim regulariter formata sunt atque ea inassueta confestim innotescentur. Tibi commendo has paginas ex Allen & Greenough.
Horae quidem cedunt et dies et menses et anni, nec praeteritum tempus umquam revertitur nec quid sequatur sciri potest. Quod cuique temporis ad vivendum datur, eo debet esse contentus. --Cicero, De Senectute