Use something that will help you measure the dough or mix uniformly.
Some possible tools to co-opt for this task:
Ice cream scoop, melon baller, measuring cups or spoons : all depending on size of course.
You should be able to get to where you can eyeball the size in relation to the tool you are using.
cooking.stackexchange.com/users/123/Ryan Elkins
If you have a dough that is dry enough (bread, cookies), the simplest method is to simply weigh your final dough, and then divide it into portions that weigh the same.
You'll be exact every time.
If you have something wetter but kinda keeps it's shape, like some cake mixes, I like to put it in a piping bag and then eyeball the portions.
It does not take too much practice to get even portions.
Though I sometimes fail at getting the correct number of portions.
If it gets even wetter, use a measuring spoon.
I suppose if you really plan ahead, you could also weigh all your containers first, and then you could weigh out the wetter mixtures too.
cooking.stackexchange.com/users/4303/Megasaur
If your aim is truly for "identical", then the only real solution is practice.
Certainly dividing your dough or batter into the requisite number of pieces before you start forming can help since you can do so more evenly than by scooping each one individually.
However, the main thing that allows people to get uniform results is less by technique and more by making the item over and over.
cooking.stackexchange.com/users/4504/Allison
I try to make items in quantities of powers of two.
Cut the batch in half.
Cut the halves in half.
And so on.
It's not as perfect as using a measuring device, but on the other hand you'll never have that remainder with which to contend.
cooking.stackexchange.com/users/2832/Sean Hart
I just made a batch of cookies (Walnut size) using a melon baller.
Recipe called for 48 cookies and that is exactly what I got.
The baller was not heaping, pretty much just filled to the top.
Makes nice round cookies.
cooking.stackexchange.com/users/46130/Janet
I get frustrated by recipes which call for the cook to shape or form little balls out of a dough or cake or biscuit mix.  The recipe will say something like "form into little walnut sized balls ... should make about 16".   I always seem to end up with some larger than others. Individuals may look and taste good but it's a shame if they don't all look identical.  Anyone got any advice?
Any advice to help me form and shape stuff more uniformly?
cooking.stackexchange.com
<baking><measurements>
cooking.stackexchange.com/questions/12618
cooking.stackexchange.com/users/373/Tea Drinker
You can use an ice cream scoop, melon baller, measuring cups or spoons to give an even size, or measure the dough by weight, especially with dough that is dry enough. For wetter mixes, try a piping bag. In all cases, your results will improve with practice. Try using a melon baller  or weighing each ball.
You can use a melon baller or ice cream scoop. Try measuring each ball to get the exact weight. With wet mixtures, use a piping bag. Practice is the only way to improve.