See the section on what sort of Djing I'm talking about before reading this bit and bear it in mind - as what I say isn't necessarily relevant if you're after other things.

Before you read on, it's not all as bleak as I may seem to make it sound.  Along the way, you should have huge amounts of fun....learning new things, noticing step improvements in your mixing, getting good feedback from your mates.  If you enjoy the music, and you've got the right aptitude and attitude, you'll almost certainly get a lot of satisfaction from learning to mix...only you'll know if you've got what it takes...

Patience & Determinism
You'll need more of this than you can imagine.  It's a very long and slow road from that first try on the decks to DJ superstardom....

At first it'll be incredibly difficult; you won't have a clue what's going on and it'll be really hard to see how things can possibly get better.  If you're the sort of person who expects to be able to do things straight away and you get bored quickly if you can't; you might be making an expensive mistake - it might be an idea to borrow or hire some equipment first to see if you're going to be able to stick with it.  You've also got accept the fact that there are many people out there who never get past this stage and realise that you too could be one of them.  This isn't bad - you've done better than most people - you've tried!  This you should feel good about.

Once you start to learn what you've got to do (but can't do it very well yet), you've then got to teach your brain to be able to do the technical bits like beatmatching without you having to think about it.  This takes time. A lot of time.  This is where the detminism has to really kick in.

If you practice incessantly, you'll go through phases where you get sick of the music cause you've been playing it too much.  Sometimes it's best to have a break for a few days, and sometimes it's best to persevere (and maybe then take a break).

A deep enough pocket

Stuff
Obviously, you're going to need some equipment to learn on.  This costs money and will burn an initial hole in your pocket.

Records
When you first start learning you'll probably find that you can mess around with the same records for weeks on end, without tiring of them at all.  This is because you're paying almost all your attention on the technicalities of what you're doing (well, you should be)....you're more concerned with working out why the mix isn't in time than you are with listening to the music that's playing.  However as you get better, you'll start to listen to the music while you're mixing.  This is where things will probably start to become more expensive.  You'll start to need new tunes; if you don't get a regular enough 'tune-fix', your mixing may start to stagnate - the music'll start to sound old and won't inspire you like it used to.  You need feeding.

Then of course if you start playing out, people expect to hear you play up-to-date tunes and not the same old stuff over and over.  There's only so many 'old-faves' sets you can play (unless you stick to friends parties), and you're not going to move further up the 'DJ ladder' if at least some of your music's not up to date.  This is going to cost, as you won't be making enough money to cover your record purchases for a long while - if ever (unless you're one of the lucky few who does reach DJ superstardom).

Sense of rhythm
I'm not sure if you really need this.  Well, perhaps you do, but not in the playing drums sense of the word. You need appreciation of the music you play...although not just an appreciation; more of a passion and a feel for it.  If you're going to have to DJ it, you're going to have to understand how and why it works so you can use its energy in your mixing to keep the momentum going and make your sets flow.

A sense of rhythm, in that you can tap your foot in time to a tune, however is necessary.  It almost certainly helps if you like to dance to what you're playing - after all that's what you'll be expecting everyone else to be doing.

Self analysis
When you practice, you really need to be able to tell what it is you're doing, why it's not working, and what you might do about it.  Until you get to a certain level, you'll probably find that you don't want other people to hear how useless you are - specially DJs.  You'd be way too embarrassed.  This makes it all the more important that you can work these things out for yourself.  If you can't tell what's going wrong (and work out what to do about it) you're not going to improve very quickly (if at all); and it's a slow enough process as it is.  Try and break any problems you're having down into smaller problems and work on them separately.  Littler problems are easier and much less confusing.  A big plus from being able to do this is that you will start to notice improvements in some of these smaller areas, even if your overall performance isn't progressing that much.  This will help a lot in making you feel like you're actually getting somewhere.

'Trainspotting'
The fact that you can spot a DJ's mistake at 100 paces doesn't necessarily mean that you're going to be able to do any better.  This is not a required skill.  However, listening to other DJs and, more importantly, watching what they are doing will help you loads.  The type of watching you've got to do is more seeing than watching (and hearing rather than listening) - you've got to do the equivalent of your self analysis; but on someone else, without the benefit of being able to hear what's going on in their headphones.  Learn what you can, watch your favourite DJs - take a bit of time out when you're in a club to do this...not too much; you mustn't forget to enjoy yourself too!  This can be really inspiring and should give you lots of new ideas and things to mess around with.

I'm stuck & self-consciousness
This is really difficult, but try not to be too shy of showing other DJs where you're at when you're still crap and you're not sure where to go next to improve.  They were learning once too, and have almost certainly experienced the same frustrations as you.  It's probably best to pick someone who's not light years ahead of you.  They'll be less likely to remember the right sort of things which will be useful to you.  You'll probably have to be fairly good mates with them - or you'll have difficulty persuading them to come round and listen to your dire mixing; they'll be more likely to jump at the first opportunity to take over on the decks and spend the allotted time giving you a first hand demonstration of their skills - after which you'll be quaking in your boots at the mere prospect of letting them hear your galloping horses and suchlike.

However, it's still really down to you - without a real understanding of what you're doing, you will find it hard to improve.