The Experience Factor
First off, let me say that, in order to use this method effectively, you gotta have a good amount of experience in the iron game. Make no mistake, when you're beginning, in other words, your first year or two of hard-core training, you have to follow a program - and follow it to the T. Most importantly, be consistent with it. If you have done this, and you feel ready to start letting your body have a say in what goes on in the gym, start incorporating a few of your own methods or changes to the program you're on. Don't start by trying a completely loose system where you have nothing on paper, no schedule whatsoever. Instead, slowly weave in little things like an extra set here and there when you feel like it, or maybe even subbing in a given exercise you just feel you'd get a great pump out of that day. Like I said, this can take awhile, so be patient and only start implementing this into your program when you know you're ready.
When you train randomly like this, your journal becomes important. If I'm in a routine I've been using for a few weeks, I have no problem remembering what I did in each exercise, or at least the big ones like bench or deads. On the other hand, the idea of training in a different order with different exercises, possibly every workout, and then trying to remember all that, is pretty tough. To remedy this problem, simply buy a notebook, and start writing. Every time I tell someone about the whole notebook thing, often their first response is that it must take forever - not true. It takes only about 5 minutes total when you get the hang of it. The key is to develop a sort of language so you don't end up filling an entire page for one workout. You develop little symbols for a good set or for which reps are partials or forced. Now don't get me wrong, being thorough is important, but you don't have to write it down if you fart on the 8th rep of your final set of flyes. I mean seriously, you should write down things that matter, but nothing more - you don't want to take an extra long rest break because you had to write something down. The best way to prevent this is to set everything up pre-workout so all you have to write down between sets is the number of reps and the weight used.
In case you're bulking, food ties in with this free style of training. I've always had a huge appetite, so I figured I'd be the last person to ever be asking the question, "How do I increase my appetite?" Well I have asked that question. When I first learned about the whole concept of bulking, and I found that large quantities of food are needed for optimal growth, I started eating more and more, but of clean foods only. To sum up, I pretty much lived off of tuna and rice. I did this for months, and got some pretty good gains, but eventually my appetite crashed. The fist thing that went was the tuna … I couldn't to even look at it. What I'm getting at here, is that you can't expect to eat clean all the time for months. Especially when bulking because you're pretty much taking in 2-3 times the normal amount of the same, tasteless food for months on end. I don't mean to eat Fruity Pebbles, nachos and pizza all day, but to use some variety to keep your taste buds from getting bored. And as for quantity, just eat whenever you're hungry. A good rule is to never get hungry, and never get full.
One big important factor before I forget … this is not an excuse to get out of squats - or any of the hated exercises. You aren't gettin' away with, "Instinctively, today I don't feel I should do squats …" Uh-uh, not gonna happen. A mindset like that only robs yourself of optimal gains. Squats, deads, chins … you know the ones I'm talking about. These are the staple exercises and should only be taken out of your routine on special occasions, like your B-day or something, and that's even pushin' it. Grow some balls, a third one if necessary … do what you gotta do to build up the toughness to go all out every time. However, this works the other way too, for something like bench, although it's definitely on the list of fundamental exercises, it's one of the few exceptions that has a reliable substitute: the dumbbell press. If, for some reason, you don't like dumbbell presses, this doesn't give you the excuse to completely throw them out the window. Bottom line: you still have to train with some variety, don't just do your favorite exercises every workout based on "instinct."
Although this training instinctively can take a little while to get used to, I assure you in the end you will be satisfied with its ease of use. It's the absolute best way I know to train I know, and I think you just might agree if you put it to the test.