Every single one of you, if you draw or paint, at least once was stuck - or will be - with this thought in your head: how should this body part/perspective/chiaroscuro be drawn correctly?
Some certain aspects of creating a picture and its composition make quite a challenge, especially (but really not only) for a beginner. This is where references come in handy. Written from a point of view of someone, who used to use them often and who does not anymore - almost at all.
The best and easiest when it comes to follow, storage and collect - photo references can be called without hesitation the most popular of this specification. On deviantART, you've got the amazing Resources & Stock Images category full of brilliant reference pictures - and there are deviants with accounts dedicated completely to providing them.
However, photo references have one main disadvantage: they're 2D. You can't walk around the referenced object, see how it behaves at diffrent angles, how the light changes around it, you can't move it to understand its nature. Even when it comes to stock packs with dozens of photos of the same object, it's still nothing like having the possibility to touch it with your own hand.
Not all photo references will suit your purpose well - some may, for example, have the shapes distorted by a wrong focal lenght, which may cause unwanted and unexpected effects in your drawing. Looking at the problem from the other side - let's say you've sketched a composition for your picture and now you need to find a photo reference - how much time will you spend on finding just the exact pose from the exact angle?
The opposite of 2D photos: take some boxes and arrange them on the floor or table to reference a city, or sit by a window of a tall building. Observe your own hand or foot. Ask your friend or family member to hold still and referencing the pose directly as it is. You can take a look at them from any side you want (well, there might be a problem with from above, if you don't have wings, but I'm sure everyone does by now). You can ask them to move that finger a little bit up, or bend the knee, or stoop, or stop smiling (careful with that). You can move the lamp around and find the nicest light angle. Or use a chair, when you don't have wings.
The basic problem about live studies: it may be a wee bit hard to sustain your model in the mid-air to reference a jumping body. This is where photography walks all over it. Also, sometimes you may not have anyone around willing to pose, or there's noone around accurate (note - a 1.50 m tall, chubby model is not the best fit to reference a 2.10 m tall, skinny character).
How about a combination of picture references and 3D? Websites like www.posemaniacs.com proove very useful to people willing to learn a thing or two about muscles. Containing hundreds of poses, it allows you to grab a skinless 3D generated model and turn it around to see it from any angle you want. It might not be the best idea, if you find such views disturbing, though. Softwares such as Google SketchUp or Poser allow you to load 3D models to tweak them into poses you need - also change the light, props and other things you'd need. akio-stock recommends artists.pixelovely.com for training figure and gesture drawing of humans and animals.
Few of these programs is free - and, obviously, you'll need to learn how to use them properly first.
Sad thing about this whole topic is that people often focus on the never-ending battle of "references vs. imagination" and tend to forget that it's not what really matters. References can help to create an imagined scene (too many forget about it), and after all, it's the final result that counts, isn't it?
The matter that should actually be discussed is credits. Whenever I see a journal full of nagging about seeing someone's drawing posted on tumblr without given credits, it always makes me wonder how many of these shouting artists used references for their pictures and didn't mention the source at all. There are, of course, stock providers who state that there's no need to credit their work, but most of them - especially on dA - have written a point in their terms of usage that credits are not optional.
Searching for reference pictures via google isn't the best idea - chances to stumble upon a copyrighted image and breaking the author's laws are really high and chances to find a stock picture without proper key words are minimal. But if you feel the urge to use such image is too strong to comprehend, ask the author for a permission. Especially when you know it's not a stock photo or when you're not sure about it.
If you are able to respect all the effort put into painting a picture,
respecting the effort put into creating a reference photo shouldn't be much of a problem.
Your own references
Solution to credits and time spent on searching for the perfect reference. Why wouldn't you make your own? Take your camera and photograph your friend all over in weird poses, take photos of everything you might need - buildings, trees, ground, clothes, clouds, animals. Remember about what I've already said about photo references - make sure the focal lenght you use won't distort the proportions, if you don't plan including these.
This doesn't refer only to photos - carrying a small sketchbook around and sketching anything that caught your eye is a brilliant idea - not only because you'll storage references to use later, but sketching itself is a great exercise.
Last, but not least. Be bright. Don't just see, observe. Whether using photos or live references, it won't do as much good if you can't do it consciously. You can reference the same pose 100 times and learn nothing, or you can use it 10 times and by the 11th you'll be able to go on without it, or with less help. Look at the picture and remember it, so the next time you'll be drawing something similiar, you'll think wait, I know how this hand goes, or I just need a reference for a small part of this one.
Observe. Waiting for a bus? Look at your hand. Look at that tiny wrinkle between the thumb and index finger. Look how it disappears when you straighten your palm. Look at the lady next to you, how her expressions change when she's talking on the phone. The best side of learning how to manage without references is that you can learn it whenever you want, just if you want.