Don’t dimension just to show everything you can see; dimension to control the sketch. Over-dimensioning a sketch often leads to an unsolvable model; it’s either not fixable, or too many things have to move at once. A better strategy is identifying what dimensions control the overall shape and what dimensions control the key features, and what constraints are most suited for the task.
Start with the overall shape. For a simple rectangular plate, you need the width and height. Center the rectangle on the origin by constraining the center, corners, or centerlines to the origin. Then you need to locate any internal features, such as a hole or slot, in a stable position (i.e., on a central axis, or constrained to an outside surface or a different feature). Don’t dimension the same surface, edge, or hole from more than one reference unless that’s a requirement for your design.
Often, geometric constraints will replace the need for dimensions. If two holes or two slots must stay on an equal level, just horizontally constrain the two centers; you don’t need to add two separate dimension lines. If two or more holes or slots are to be equally spaced from a centerline, just apply a symmetry constraint; you don’t need separate horizontal dimension lines to locate each. Other useful constraints are parallel, perpendicular, coincident, concentric, and tangent. These define what geometric behaviors the sketch should have while leaving the dimensions to define size. That makes it more clear how the sketch will behave.
Practice this on a simple plate with two identical holes and one slot at its center. Dimension the width and height of the plate, constrain the center of the plate to the origin of the drawing, and define the diameter of the two holes only once. Constrain both hole centers to a vertical centerline with a symmetry constraint, then dimension the distance from the hole center line (only one dimension is sufficient). Dimension the slot from the hole center line with only one dimension, and dimension the length of the slot. Change the width dimension in the top right; the features should all move in relation, keeping the same alignment. You don’t have to add another dimension line just to show what the change was.
When you add a new dimension, often the first thing to realize if you get a conflict is that the model already knows that fact. When a line is constrained to be horizontal, don’t need to show a linear vertical dimension on both ends; if it’s horizontal, the distance will be zero. Two identical circular shapes don’t need to be dimensioned as two separate diameters if the two circles are already constrained with the “same” constraint. When the model rejects a dimension, check if there are existing constraints. This often happens not because the geometry has gone awry, but because the sketch has conflicting requirements.
Finally, make sure that when you read the dimensions, the design is unambiguous. You should know the general size of the model, be able to locate all of the key features on the drawing, and know when similar elements are to be treated differently or equally. Also, try changing the value of any important dimension and confirm that there’s no unexpected motion. If the sketch is fully-constrained, it will still be easy to understand the dimensioning scheme, and the resulting geometry will not look like a distorted version of what it should.