I know, I know everyone knows how to merge multiple layers, but do you know how to do it without loosing each individual layer?
Here's how.
Once you have an image open that you wnat to merge layers with, click on the top layer (figure 1), next click on the add new layer icon (figure 2), then select all of the layers that you want to merge including the new one you just created (figure 3). Press and hold the alt key while you select the layer option flyout menu and select the merge layer option (figure 4).
figure 1

figure 2
figure 3

figure 4
The way I use this a lot is that I will take the top layer that everything gets merged into and drag it to the bottom and either add effects like outer glow or make a nice drop shadow like in the after picture below.
Before


After