Lambda the Ultimate

There has been a Language Design 101 with the comment: "Let's not to turn this into a thread about language implementation tips. We'll do that one later." I have not found this "later" so far. Here it is: How would you implement your language?

Of course, the requirements might be very different. Quick implementation to try out programs as soon as possible? Using LLVM for optimized code generation? Interpreter vs compiler? Just a language extension or a clean-room implementation? So the tips should clarify their context.