Proper way to use sentence-final particles (like 啦 or 喽) in spoken Chinese
Source: http://en.wikipedia.org/wiki/Chinese_exclamative_particles

But use of exclamative particles is highly informal, and it is advised that they not be used in formal documents or academic papers, unless it is specifically required to do so (such as the case of narrative telling). Some common examples are shown below.

了   le  modal particle intensifying preceding clause / completed action marker
呢   ne  question particle for subjects already mentioned
吧   ba  modal particle indicating polite suggestion / ...right? / ...OK?
哦   ó   oh
啊   a   modal particle ending sentence, showing affirmation, approval, or consent
啦   la  sentence-final particle, contraction of "了啊" / follows after each item in a list of examples
呀   ya  "softening" particle used in questions; may also be used like 啊 after a vowel, expressing surprise or doubt
嗎/吗 ma  question tag / not for open-ended questions


I should add that each of these characters, with the exception of 了, have the 口 radical. This suggests that they should only be used with the mouth (oral communication) and are informal. They are not generally used in formal papers/documents.

So, in summary, there isn't a "proper time" for when they should be used in colloquial conversation, just only when you think it will give that added emphasis you're looking for... whether it be cute (我们一起吃饭啦！) or demanding（你跟我吃饭嘛...)。 Just like in English how there aren't really specified times you should be using aww you guys are so cute, or pfft that was lame, or gee could ya take any longer??, it's just there to add some flavor.