There are numerous techniques, which I will outline below.
1. The Good Ole' 'Stuff-It-Under-Your-Top' Technique.
This is the classic technique which is used by most people. Quite simply, you buy what you want to buy- and then hide it in pockets, under clothing, in shoes... basically anywhere where it might fit. If you are going to do this, I thoroughly recommend taking an obese person or big-breasted girl with you, as staff very rarely have the guts to say "Are you hiding something under that top?" -- because then your burger-eating/breastily-gifted friend can use the "are you calling me fat/are you staring at my boobs?" card. I should warn you, there isn't an actual card for this, it's just an expression.
If you don't have any obese or big-breasted friends, they can often be found passing by in the street, and it's quite possible they enjoy movies.
2. The 'That Person Over There Said It Was Okay' Technique.
This is very simple. When you enter the cinema, you check out the name tags of workers on the way in. It's best to find somebody who looks like they have authority. These often are women in their 50's who look like war criminals, they have names like 'Anne' and 'Magda.'
When you get to the final check point and hand over your ticket, there is likely to be a seventeen year old who's a bit of a jobsworth. He delights in saying "Sorry, you can't bring food in here, it's against policy." When you respond with, "A woman called Magda said it's okay," you will see him visibly begin to shake, as his eyes fill up with fear. Not only is he scared of the Nazi-like manager, he assumes no customer would ever attempt such a ruse, and will promptly let you in.
3. The 'I've Got A Disease' Technique.
In recent years, I've managed to build a bit of a conscience and don't really use this technique any more. But basically, what you do is demand that you can take your specific foods in because you have some kind of illness. Diabetes is a common one to use, as most people know, vaguely, it has something to do with sugar, so they just assume you know what you're talking about and let you in with your fizzy cola bottles and yards of liquorice. Another common thing used to be to make up illnesses-- but often you find that the low-paid staff also happen to be at university studying to become doctors. This is troublesome as not only do they diagnose you on the spot, but they also delay appointments by up to four hours.
4. The 'Supreme Confidence' Technique.
This is the technique I use the most today. It's basically just positive thinking at work. You believe you are entitled to take your food in. You put your mind in a position where it is an absolute right. It's a human right. You make up the rules. You are the KING OF THE CINEMA. This technique is great as you can literally hold your goods in plain view of all staff--- but you are so freewheelin' and happy about what you're doing that staff will assume there is something special about you, or they'll assume you have a special reason why you're allowed to bring your pizza and coke into the building.
Make sure you are completely confident and happy throughout. When you get to the final checkpoint, happily hand your ticket over and say to the dude, "Hey man, how's it going?" - as you chirpily pay interest in him and exude complete confidence, there is no way he's going to turn you away!
5. Except for the times he does turn you away.....
And it's all your fault. A little drop of confidence, where you just take your eye off the game for a split second is all it takes. "Sorry, but you can't take your food in," is his standard reply. Now, there are many ways to deal with this. I've tried them all, from "Okay, after the movie - you can take my girlfriend out, how's that? Now will you let me in.." to "Why the fuck do you care about these stupid fucking rules when you're earning £5 an hour? What difference does it make?" to the truly lame, like "I'm just taking it in, I'm not going to eat it..."
But, after much trial and error, I've found the best way is to show innocence. "Oh, I didn't realize. I'm sorry..." -- the dude will then reply, half-guiltily, with "sorry, it's not my rules." Then, what you do, is stand about two metres away from him and begin eating your food. As you do, you start up a conversation with him, ask him what he's studying, what he does in his spare time, what movies he's seen recently. This is like a complete magic act, because as soon as you do, the cinema dude will say, "okay, look, you can go in. Just this once, but in future..." And then you're in with your food, drink, three course meal, everything! You get to take it all in. Magic.
6. The 'Extremely Hot Girl' Technique.
This is a superb technique that works, without fail, every single time. The problem is, you need a really hot girl. In principle, this is fine, but whenever I try to take one to see a movie they are unfortunately "really busy" or "taking time off from movie-going right now." However, sometimes I do get them to come with me, which coincidentally tends to be when my friend Jed, the guy with big muscles, comes along too.
What you do is this: you give the girl everything you want to take in. Guys who work in cinemas tend to be dweeby looking, under-sexed geeks. The only girls that talk to them are their sisters. The hot girl goes up to the lamest looking geekster ticket-dude she can find and says, whilst touching his arm, "I really need to take this food in to my seat. I know I'm not allowed, but I really want to. Can you help me out just this one time? Do you have the power to do that, to help me out?" and then she touches his arm again and smiles.
Seconds later, you will all be inside with the food, and the geek will have probably given her free popcorn and drinks too.
In Summary...
There are countless techniques that can be used for sneaking things into the cinema. I'd love to hear things you've tried over the years -- whether they succeeded or failed. It might also be an idea to develop new ideas, as we continue to outsmart the dull looking cinema staff.