In my life, I tend to follow a general trend of what I'll refer to as "balanced spontinaety"... that is, I prefer to be spontaneous in the things that I do, with a certain amount of planning when appropriate. Basically, if I can avoid actually scheduling something, I will... though I acknowledge that certain things simply MUST be scheduled (hairstylist appointments, for example, which should never, ever be missed ;)). This means that when someone says something like "Hey, do you wanna get together tomorrow at xx:xx time", they tend to get a lukewarm response, because I hate planning.
The whole thing focuses around the acknowledgement that I am a moody person. With every moment of every day, my mood, opinions, thoughts, desires, interests change and evolve... and more often than not, certain friends are more compatible with one mood than another. There are certain friends that just don't go well with "bitchy Chris"... or certain people that I know it's unwise to be around when "flirty Chris" is visiting... there are people that just don't work with "relaxed Chris" because their energy level is through constantly the roof... or sometimes "Therapist Chris" doesn't feel like being around long enough to deal with the woa-is-me people (these people are just as much my friends as any other, and I cherish them all the same, but that doesn't mean I'm always in the mood to handle the attitude).
Now, all that being said, the reason I don't like scheduling time with most people is that, when that time rolls around, I have no idea what mood I'll be in. So, if I schedule to do something with someone at say, 3pm, and they're not compatible with "bitchy Chris", (eg., a woa-is-me person), I have two choices: cancel and be a flake, or go and create some form of misery... misery for them if I let my mood be, or misery for me if I have to fight my mood (and I'm sorry, but yes, sometimes I really do enjoy hanging out with bitchy Chris :P)... neither of these sounds attractive. This brings up the possibility that my mood will change to be something compatible with the friend as I'm going to see them or they're coming to see me, but considering the array of moods out there, relying on a specific one to fall out of the sky is sketchy at best, and I prefer not to rely on it happening.
So what's the answer? Well, first, be understanding if I'm hesitant to actually schedule something. It's not about you... or wanting or not wanting to be around you. It's about me, and the acknowledgement that neither one of us might want to deal with whichever Chris happens to be visiting at the time. Second, if you don't want to live your life on my tentative schedule (which I totally respect), then give me a call when you get close to ACTUALLY wanting to do something (30 mins notice is usually good). I can usually predict my mood within that timeframe and will give a go, no-go response that we can hopefully all live with...and if I want to sit at home and wallow in my own misery, so be it :P
Fundamentally, I'm an interrupt driven person. When you want my attention, interrupt me. I think that consistently getting a more enjoyable time less frequently results in a better friendship experience than getting a questionable friendship experience more frequently.
(Note: as the evening wanes on, the likelihood that I'll be willing to go out fades with it... so if it's 10:00, even on a friday/saturday night, don't be surprised if I don't want to go out at all. At that point, sitting at home in front of my computer, I can hear my bed calling me... and it's very, very persuasive :)