As I've said before, I'm not too keen on doing the nuts and bolts of a constructed language as part of my worldbuilding. Well, more correctly, I think it's a great thing to do, I just don't have the skill or patience to do it.
However, playing with idioms and slang? That's always fun. And, I think, an important thing to do. It seems like it was the underlying structure behind Douglas Hulick's Among Thieves, even to the point of bringing up the details of his Thieves Cant in the foreword. (I especially like how he admits that he combined historically accurate slang with stuff he just plain made up. I approve.)
I made up plenty of slang for Thorn of Dentonhill and the other books set in Maradaine. I even made a point of tweaking it a bit so that slang from the gangs in the Aventil neighborhood would be different from the slang of street kids over in Seleth and Keller Cove (in Holver Alley Crew). Some of the things came from wanting a slang word for something, and wanting to avoid our own word. Street kids and gang members would have a term for constabulary officers, and I didn't want "cop". I tried to think of what they might call them. Since patrol officers are all armed with handsticks (and might use them a little too freely on some street kids), the term "sticks" made perfect sense to me.
That, I think, is the key to doing this sort of thing: does it feel like a term or phrase that evolved naturally? Can someone reading it parse it a figure out the term from context? If so, then you've got a winner.
Idioms are a bit different. I wanted Druthal to have idioms that didn't necessarily apply in English, or meant something that the idiom in our language was too modern or culturally specific. Sometimes I look to idioms in other languages (though usually Spanish, for obvious reasons) and translate them literally into English. Case in point, one of my favorites in Spanish is one common response to ¿Cómo te va? ("How's it going?", or more literally, "How to you does it go?"), which is Va a la patada. Idiomatically, you're saying, "It's going badly", but literally you're saying, "It goes of the kick." In other words, "Life is kicking my ass." Thus, "the kick" could become slang for any kind of ill fortune.
A brief political aside, for those interested: I'm all for donating to Planned Parenthood, and if I had something I could offer as an incentive to get others to do it as well, I'd do it. But I don't. (At least, I don't think I do. Open to suggestions.) Thus, I'll just point you toward some writers who are. Buy John Scalzi's eBooks this week, and his profits go to Planned Parenthood. Also, Amanda Downum is offering signed copies of her highly-anticipated (by me, anyway) Kingdoms of Dust to the first 15 qualifying PP donators. Have at either one, if you are so inclined.