Early times
Artists have been using mechanical aids for centuries. In the 15^th and 16^th centuries, for example, a Master would draw on paper his complete design for a fresco, then his assistants would transfer an area of that design to damp plaster sufficient for the day's work. This they did by pricking holes along the lines of the drawing, holding the paper up to the plaster surface and then "pouncing" (pummeling) the lines with muslin bags of soot or carbon. The artist could then continue his work assisted by a representation of his drawing in the form of closely spaced black dots.

Using this machine, sighting from a fixed point, he would look through a grid of strings and transfer what he saw onto his drawing surface, which carried a similar grid.

Replace the 3D object with a 2D sketch or photo and you will see that Dürer was using a precursor of the now common grid method of sizing up (or down).

You may have heard it suggested that the use of such contrivances in some way diminishes the "art". I strongly disagree with this. Had Dürer had the use of a camera to convert his three-dimensional forms to more manageable two-dimensions, he would have used it.

Pre-draft Planning —To fail to prepare is to prepare to fail
There are only so many changes you can make to your drawing before damage results so it's preferable to refine your study during earlier stages. I'll assume here that you begin (as I do) with a small, manageable line-drawing. It is at this stage that major changes and decisions are made, the composition finalised and problematic areas sought out and resolved. If you are basing your drawing on a photograph, you may be tempted to size-up and transfer your photo to your working surface in one step but I would council against this approach - you will also be transferring potential problems that occur naturally in the photo but will destroy or unsettle your finished drawing. The removal of the background, for example, may show up distracting (natural) lines that run through your drawing that now upset its balance. Using the intermediate step above will allow you to fix optical errors and create a new man-made "natural" balance that adds harmony and pleasure.
So, you're happy with the result and now you need to enlarge your line drawing and apply it to your final drawing surface — here's how...

The Grid method of resizing
When sizing-up from a sketch you can lightly draw a grid of regular lines on the drawing itself or (as with photographs) you can use a suitable, fine pen to draw your grid on an acetate sheet. The acetate method is preferable as you have a permanent, ready-to-hand grid for all occasions. The smaller you make your squares the more accurate your copy will be. I found that a grid of ½-inch (1cm) squares was quite adequate for a 6"×4" photograph.

But I thought Mike advised against sourcing direct from a single photograph? True - but you can size-up twice! First translate your photograph into a larger (but manageable) line-drawing then size up the line-drawing to the final dimensions required. Why? Your line-drawing provides many advantages. It gives you the opportunity to make alterations. If for example, you are drawing a dog, this is the ideal time to incorporate changes of expression, open or shut the mouth or move that awkwardly lolling tongue. It also removes all three-dimensional shading, which makes applying the lighting as you want it far easier. But above all, it greatly increases your understanding by getting you in touch with the form itself, not just its photographic representation. OK, sermon over! Lesson time...

Advanced Grid Method
stage 1
This is an advanced variation of the method shown above and has a few major advantages - no grid is drawn on the paper, guide-lines are easily erased, and immediate and accurate referral to the guide-lines is allowed even after they've been erased.

stage 2
This is the method I used for almost all of the 52 head-studies I drew for my open edition print collection.

stage 3
Using Graphite "Carbon" paper
To complete the advanced method, slip the carbon paper under your drafting paper and (with your hand again resting on a piece of card) re-draw over every line. I used a blunt darning needle in a clutch (or drafting) pencil holder for this but a hard-lead pencil will do the job. You can safely remove the carbon paper at any time and check your progress because the top line-drawing sheet (if you've securely taped it in place) will always accurately return to the same position. Indeed this method of working has much to commend it...

When working with this method my practice was to erase the guide-lines, in the area in which I was about to work, until just the merest shadow remained (by rolling wall-putty over them) - if you need your guide lines back - drop down the line-drawing overlay.

And finally DON'T BE A SLAVE to your guide-lines... at best they provide accurate positioning, at the worst they are simply wrong - you draw what looks right, not what the photos say it should look like.