I know it's very common for Chinese speakers to add special particles to the end of their sentences to enhance the tone a little bit, such as adding 啊 to show excitement or 啦 to sound cute.
Just to show a few examples, I've heard native speakers say:
我们去吃饭吧 (The proper way of saying it)
我们去吃饭啦
我们去吃饭喽
我们去吃饭耶
Besides just throwing any random particle at the end of sentences for fun, is there a more proper explanation of how and when they should be used?
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.
I know that in Japanese there are also sentence ending particles that are both gender-specific and can change the tone of the expression. So it certainly isn't random, and you need to be careful when you use which so the correct meaning is conveyed.
From what I hear and am familiar with (from a Taiwanese perspective):
我们去吃饭吧 - shall we/why don't we go eat (inviting or encouraging)
我们去吃饭啦 - lets go eat (asking but more imperative)
我们去吃饭喽 - we're going to go eat (telling someone what you are going to do)
我们去吃饭耶 - are we going to go and eat? (uncertain or questioning)
I am Chinese. I can feel some mistakes in the previous answers
Sentence-final particles can be very hard to English speakers, because they never exist in English.
我们去吃饭吧 - Normal suggestion
我们去吃饭啦 - To inform(the listener is excluded. May be past aspect), with some little sense of happiness. Equivalent to 了啊
我们去吃饭喽 - More happiness, even some exciting, comparing to 啦. Present aspect only
我们去吃饭耶 - feel very happy about the (going to) dining
Only long-term immersion can make you master these sentence-final particles
耶(though it's often heard in Taiwanese girls' talking), as it indicates you're pretending to be cute. It sounds gay.