Sometimes it can be desirable to position yourself on foot quite precisely; for example in setting up a very precise grenade throw. You can potentially do this by 'down-aiming' at a particular ground marking you've learnt. By down-aiming I mean having your gun elevation as low as it'll go, and having the relevant marking targeted. The pistol and sniper rifle are especially recommended for this because they have nice small reticles, as well as zoom capability if you want it.
Be aware that your aim will be slightly ahead of your position - by almost exactly 3 pistol reticle radii in fact. So if you have a pistol reticle (unzoomed) and you imagine another one below, touching it, your position would be at the bottom of that second one, near enough.
Making fine adjustments
If you're not quite on target with your down-aim and you need a small shift to fix it, even a brief push on the movement joystick can cause you to overshoot; and then if you try to move back you may overshoot again, and so on. Depending on your joystick finesse, that can be clumsy, taking many attempted nudges before you get the aim you want. Instead of suffering this, you can use 'jump-moving'. Jump, and do a brief movement push at the top of your trajectory or thereabouts. The push will move you a much smaller distance than if you'd done it while on the ground. So, with this technique you can make much finer movements.
One other method of making a small movement is to move while crouched. But personally I find that quite awkward, because you have to operate the movement joystick while keeping it depressed. And although you do move more slowly, it can still feel too clumsy.
Positioning using a niche
Sometimes you might not need the fiddly business of down-aiming. A much more convenient way of positioning yourself precisely - when available - is to wedge yourself into some sort of niche, giving you the exact same position every time you do it.