Just choose a darker shade of your color for the color patch on the far right, a very light shade for the color patch on the far left, and a bright shade for the patch in between, as shown below.

Green rounded rectangle gradient button

2. Add a dark green, 1px stroke layer style to your button, as shown below.

Adding a stroke to the button

3. Add an outer glow using a bright version of the button color, as shown below.

Adding an outer glow

4. Select the button layer in the Layers palette and duplicate it using Control-J (Command-J on a Mac). Right-click (hold Control and click) on the new layer and select Clear Layer Style from the menu that appears, as shown at right.

Clearing the layer style

5. Change the Opacity of this layer to 50%, and double-click on its color patch to open the Color Picker. Set the color of the shape to white, as shown below.

Changing the color of the shape layer

6. Now, click on the vector shape for the same layer in the Layers palette. Using the Direct Selection Tool (A), click on the bottom line of the rounded rectangle and drag it up a little bit, as shown here. (You might need to zoom in for this.)

Editing the vector shape

7. Still using the Direct Selection Tool (A), click on the bottom-left anchor point so that you can see the handlebars of the point (zoom in if you need to). Click on the bottom handlebar, hold down Shift, and drag the handle up to curve the corner.

Curving the corner

8. Repeat step 7 with the bottom-right point. The sides of your shape should now look like those shown in the image below.

Shape after modifications

9. With the Direct Selection Tool (A), click and drag the mouse to make a selection that captures all of the bottom points, as shown in this image.

Selecting multiple points for editing

10. Hold down Shift and drag the selected points up to make a thin strip, as shown at right -- this is our highlight. Fine-tune the movement using the up and down arrow keys if you need to.

Making the highlight strip

11. Select the highlight layer in the Layers palette and duplicate it using Ctrl-J (Command-J). Select Edit > Transform > Flip Vertical to flip the duplicated layer.

12. Use the Move Tool (V) to move the flipped highlight to the bottom of the rectangular button, as shown here.

Adding bottom highlight

13. We're almost there! Now duplicate the button shape layer and name it middle highlight. Change the color of the shape to white.

14. Select the bottom anchor points of the middle highlight shape and move them up to the center of the original button shape. Change the opacity of this layer to 25%, as illustrated in this image.

Modifying the third highlight shape

15. Add a text layer immediately on top of the original button shape (beneath the three highlight layers) and type your text. If you like, add a drop shadow for an added three-dimensional effect, as I've done for the completed button here.

The completed plastic button