You might consider "effusive", or "gushing":
effusive (adj): extravagantly demonstrative of emotion; gushing
gushing: (v) to act or utter in an overeffusive, affected, or sentimental manner.
(n) an extravagant and insincere expression of admiration, sentiment, etc
Or, particularly in a situation where you're clothes shopping and the sales clerk tells you "you look amazing in that!" no matter what you put on or how it looks on you, and in general is overdoing it with the attention and flattery, you could say he is "fawning", "obsequious", "unctuous" (my personal favorite, it just sounds slimy), or any number of synonyms.
unctuous: (of a person) excessively or ingratiatingly flattering; oily. Affecting an oily charm.
Finally, there are a set of over- words which emphasize different aspects of this annoying behavior:
overeager: excessively eager or keen
overbearing: domineering manner or action; bossy, pushy
overweening: (of a person) excessively arrogant or presumptuous
EDIT: Ok, I just went back and read the comments. Are you simply looking for
The sales person was way too insistent