MOOP!

My general tactic has been to either go "Hey, FYI, I'm going to be in [CITY] on these dates for another thing! Want to meet up?" or "Hey, I'm strongly considering making a trip to [CITY] this fall, maybe in late October. What're you up to around then?"

Tactic 1 is mostly when I'm already coming for some other reason. I might already be staying somewhere, or I might have somewhere to stay for some of the time but be considering staying a couple extra days with friends if the friends offer. (Although I'll usually make it clearer, in that case, that I'm coming for [THING] but that the trip's dates are flexible beyond that.) It's usually more "Let's meet up!" than "Do you have a couch?", but that's flexible.

Tactic 2 is when I would really quite like to stay with a friend. But that gives them the option of saying "Hey, awesome! My couch/floor/spare room is yours if you want it!" or saying "Hey, awesome! Let's have lunch or something!" or saying "Oh, boo, I have relatives already visiting in late October. Otherwise I'd totally offer to put you up." If they don't offer couch space, I know to either see if I can (and want to) swing a hotel, or if I have other friends I can stay with, or how I want to handle that. And if they have a schedule conflict, I can revisit my schedule and see what's worth rescheduling for.

It's been a while since I visited somewhere where I didn't either have hotel plans (for a con, on a family trip, etc) or a close enough friend that I knew we had mutual stay-on-my-couch-any-time consensus. But I'd float it the same way with close friends, really; I'd just probably toss in some kind of "Any chance of couch space? :D?" rider.