This tip is one of those aha moments when you see it for the first time, but in no way means what you were doing before was wrong, it just means it perhaps wasn't as efficient as what I am about to show you. The common modeling question here is what is the best way to center a hole on a face of a model. We'll keep it simple for now with a rectangular shape but the last method also works wonders for irregular shapes.
Method 1 - Linear Hole Placement.
This is pretty basic way of placing a hole and is a pretty good method. Start the Hole command and choose the Linear placement option if it is not already selected. Choose the face of the model to place the hole. The next step a lot of users miss or just haven't used really well. In the first reference edge selected, hit the flyout and choose Show Dimensions and then select the feature that has the size controls for your model.
You can now link values from the other sketches and features used to control the placement of your new hole. In the image below, the 30 value is linked into the Reference 1 box simply by clicking on it and then dividing it by 2 in order to center it on the face. Lather, Rinse, Repeat for the second reference for the hole placement.
Method 1 Downside - Requires linking of dimensional values to stay put where you want it.
Method 2 - Sketch Placement.
Method 2 Downside - Requires a new sketch for a simple feature and additional geometry formatting and perhaps dimensions as well linked to other values.
Method 3 - The bomb diggity best method.
With this method, we are going to place a Work Feature on the face that will always stay in the center of the face regardless of geometry change on face itself or the rest of the model.
To place the Work Point using just the Work Point command and not the menu flyout, start the command as usual from the Ribbon or use the period key for the shortcut. Right click and choose Loop Select to enable that type of selection.
Once you select the loop of the face the command will place a Work Point on the face that will always stay at the center of the face even if the geometry changes.
From there start the Hole command and choose the On Point placement method. Select the Work Point and then chose a direction reference for the Hole. This can be an edge or as I have used it here, one of the Origin Axes.
Method 3 Downside - There is no downside to awesomeness.
This last method also works on faces with no straight edges such as spline shapes or complex face profiles. Essentially, the Center of a Loop of Edges will always find the centroid of that face to place the Work Point.
Here's hoping this look at a simple modeling placement makes your modeling more stable and easier to modify down the road!