I use lol pretty often with my friends, and add more "ol"s to the end if it's really funny, so it ends up becoming lolololol. When things are hilarious, I end up using rofl (rolling on floor laughing), even if I'm not actually rolling on the ground =P Another term that was pretty popular with my friends back in high school (we don't use it anymore) was "roflcopter." I think my friend coined that from helicopter-parents since she had that issue and always had to scream "COPTER COPTER COPTER" in the chat panel.
Since I switch between English and Japanese keyboards between friends, I found that the emojis are more versatile and accessible than emoticons. With the emoticons my friends and I use, we tend to use what we can draw out with our given keys (the provided pictorial ones were either super ugly or with the really old phones, would take up too much space data-wise) like :D ^_^ :O OTL @_@ We don't usually use emoticons unless we're sending something sarcastic or just being annoying and spam the crap out of each other with useless emoticons =P
As for how to make texts more casual, I guess it's just how we would normally talk is portrayed in our texts. I only text my friends and leave professional talk to emails as with the space limitations of a text, you would tend to shorten your speech, which I feel is too casual.
I normally text people in their 20s or younger, so I don't know much about people older than me, but my mom does use "haha" occasionally.
Just a little something extra: Another thing that was quite popular in high school, since we were busy trying to be annoying to each other was 1337 speak. Th1s w0uld b3 @n 3x@mpl3 of 1337 sp3@k. You'd probably see this in gaming forums, and I wouldn't be surprised if middle and high school students still use it today.