Your best mate is making big changes in their life - is there still going to be room for you in their busy schedule? That depends on how good your relationship is, what the changes are, and how much you both want it to work out.
They're moving away
If they're moving to another part of the country, or even overseas, it isn't the end of the world. You can still stay in touch by phone, email, letter and so on, no matter how far away you are from one another. Yes, you will probably miss those regular nights out, but that doesn't mean you can't meet up for weekends or holidays.
Who knows, maybe they'll end up somewhere interesting or exotic and it'll be an excellent excuse for a trip to see them. As a goodbye present, you could get them a phonecard or some stationery with stamps and your address already on them, to show that you want to stay friends. Even if they're out of contact for a while, they'll get back to you eventually if the friendship is still strong.
They're changing jobs
Going up the career ladder means that there may be less time for your friend to hang out with you, or they might be under stress or change due to different experiences that they're going through. Put the emphasis on quality rather than quantity, plan ahead with that precious leisure time and do something you both enjoy rather than staying in and ordering yet another pizza in front of the telly.
They're getting married/moving in
Newlyweds and otherwise loved-up pals are notorious for dropping off the social map for months on end after they've got it together. But it's healthy to keep friendships going from before you got coupled up, so remind your mate of this, and don't be scared to ring them up and arrange to do something. Be persistent, they may be stuck in a boring staying-in rut and need a little bit of a shake-up. If you're coupled up too, and you all get on well, have a double date.
They're having a baby
This one needs a bit more tact. The baby is likely to be the most important thing in your friend's life so far, so you can't start whinging that you're not getting enough attention. Having a little 'un also means your mate is going to be exhausted and sleep deprived too. They will need a good friend though, so you can help out in practical ways, offering to look after the baby while they have a well earned sleep, or taking your pal out for a night on the town while their partner or another mate babysits.
When the newborn arrives, don't automatically rush out to get something for the baby like everyone else will do. Treat your mate to something just for them, to say 'well done', and remind them that their identity is more than just 'Mum' or 'Dad'.