The best place to start retouching a portrait is round the eyes. The eyes are the most expressive part of the face, and the area that most people look at first. Take your time retouching this area, because it is here where mistakes will be most noticeable.
After choosing a narrow, soft Healing Brush, select a place on the portrait with clear unblemished skin, preferably near to the eye area. Hold down the Alt key and click on this spot to select it as the sampling point. Next position your cursor at the end of a wrinkle, click and drag a short distance along it. Photoshop takes the base colour from your sample area and applies it to the target, blending it in with the surroundings and hopefully hiding the wrinkle line.
Because of the unique way the Healing Brush works, unlike with the Clone Brush you don't have to keep replacing your sample point. As long as you are working on roughly the same colour it will blend in perfectly. Continue to work using short strokes, varying the size of the brush as necessary using the square-bracket keys. You can remove all the eye wrinkles this way and smooth out the skin texture under the eye, but it's a better idea to leave a few small lines. If the result is too perfect it will look fake.
Next, move on to the mouth area. Again, use a narrow soft Healing Brush to cover the smile lines around the mouth, but don't overdo it. You just want to erase the skin creases in the folds, not flatten the whole mouth area. Take care that the blending matches the surrounding variation in shade.
Again, subtlety is the key. If you remove too much the result will look unnatural.