Fashion is architecture: it is a matter of proportions. -Coco Chanel
If your coat is too long, people will think you are too short. Your clothes are at fault, but you'll always get the blame.
First, the easy part:
You basically have two sections to clothe...your legs and torso. A few other things will factor in like hair and shoes but we'll get to those in a moment. If your legs look too long then your torso may look oddly short or you'll look very lanky. The reverse is also true where a long torso will shorten your legs, forcing you to look squatty and top heavy.
So that's the basic idea: find an even 50/50 ratio between your legs and torso so that you'll look balanced and taller. Since most other men are so poorly dressed not only will you look generally better, your proportioned outfit will correlatively add that much more to your stature.
Where it can go wrong:
1. Hair - Your clothes build a shape and the hair finishes it off. If you look unkempt above the collar it doesn't much matter what you wear, people aren't going to be thinking about your clothes. But more than that--the shape of your hair can counterbalance an overly round face and add symmetry to your presentation. Don't discount the importance of a good hair dresser who can make valuable recommendations about your hairstyle that don't rely on the latest fad.
2. Collar - There are two basic kinds of collars: point and spread. Use these shapes beneath your face to add symmetry (just like your hair on top). If you have the round pizza face, consider using a point collar to add some length and, conversely, a long face will benefit from the spread collar to open up your presentation.
3. Tie & Lapels - Proportions are horizontal as well as vertical. It seems like most ties out there are too wide for the men who wear them. Don't get me wrong, I'm not an advocate of the ultra skinny tie or the bolo, but I do think that the size of the tie should be proportional to the size of the man. If a slim guy wears the normal 3.5"-4" tie it looks like a long bib. Try to find even proportions between the width of your lapel and the width of your tie. That way there will be some evenness side to side (Lapel-Shirt-Tie-Shirt-Lapel) and just avoid items very slim or very big.
4. Sleeves - Length is key. You don't want your sleeves bunching around your wrist (where you'll be perceived too short) and neither do you want your wrists to show (where you'll be perceived too lanky). Remember what I said earlier about your clothes being at fault but you bearing the blame? This is a great example about people perceiving you a certain way even though it may not be true. If you're wearing a blazer, aim for 1/4"-1/2" of cuff to show. Less is conservative and more is fashion forward.
5. Pant rise - Simply stated, this is how high you wear your pants. Dad jeans that encroach on the navel aren't bad just by association--they give you a long abdominal pannus that unnecessarily cuts your body in half with a very short torso. And they make your butt look flat.
6. Pants break - When you go to the tailor or buy a new pair of pants and they ask you what kind of "break" you want you'll now be empowered to make that decision confidently. You want a slight break. What does this mean? The break is the amount of fabric bunched up at your ankles. If there is a lot of fabric, you'll look like you're wearing your dad's pants and you'll look hopelessly short. On the other hand, if you ask for no break you'll get a very clean line that looks great!...until you walk around and your pants start riding up your legs like you're scared of the nearest puddle. A slight break (or quarter break) is literally a small break in the ironed crease down the front of your trousers that enables you to live in a 3-dimensional world with pants that look like they actually fit. Oh...and no cuffs. That's only for the tall guys.
7. Shoes - Throw away any shoes in the closet that have a flat toe. You've worked so hard getting to this moment: you've fixed your hair, bought a suit with the right length, picked out a tie of appropriate width and argued with the tailor who thinks you need need a full break, cuffed pant. Don't throw all that away on shoes that aggressively punctuate the end of your silhouette. Shoes should have a round toe that help you taper off and yet again extend your height. The shoes don't have to have a European elf-point flair, they just need to be round enough to be a seamless end to the bottom of your well appointed stature.