Decide how much of the paper/image you wish to see including signatures &c. Consider the width and colour of any window mount (mat /passe partout) you wish to use. Traditionally watercolours might have wash lines or other decoration on the mount. If you want to see the edges of the paper you might consider having the image float mounted with a gap around it and spacers or mount to hold it back from the glass; this will require a deeper moulding or box frame.
When choosing a frame think about the style, period and feel of the image, what you like and what might suit your interior/purpose. Keep it simple. Antique frames might be available and gilded frames can be ordered. Remember that larger images are going to need stronger frames to support them.
If you are worried about fading or discolouration of the image; UV filtering glass and acrylic is available and recommended for watercolours. If reflections might be a problem modern anti-reflection glass can eliminate them.
Restoration /conservation of watercolours is difficult and not always possible but, expertly done, can produce excellent results.
Please see our conservation page for further information