Eyes are the windows to the soul as the saying goes. We are drawn to eyes to determine intent and reaction. People instinctively scan others for imperfections so that when the eyes in a portrait are not accurate or soulful we know immediately. I sometimes spend as much time on an eye as would for hair for the reasons I stated. We all can accept a bad hair-day but not an imperfect eye; we stare at that.
My tips are:
Use your photo reference. Study it.
Keep your reference right beside your painting. Mine is taped to the painting in the example.
Make sure your drawing is accurate. Look at shapes, subtle curves, overlaps.
Work with a magnifying light; get in close.
Spend the time and don't settle for good-enough.
A couple of tips from this piece:
In this pre-painting you will see the use of liquid frisket. I use it for layers rather than to save it for preserving those final whites. Frisket creates a hard edge that stand out against the softness of a wet in wet watercolor.
Second tip is one of opposites or contrasts -
warms against cools;
darks against lights;
large against small;
never the same shape
If at first you don't succeed...
The best advice an artist teacher ever gave me was - "It's only a sheet of paper."
Simple advice but sometimes hard to follow. We create a painting and we're proud of it; our mind saw it, our eyes guided our hands. Stroke after stroke we built it into our painting! But you know, sometimes we miss the mark; sometimes it doesn't excite us or others. That is when you remember - "It's only a piece of paper."
Look at that first attempt - what didn't work or what was over-worked. Analyze it; come up with a new plan - a new color pallet or a new composition or a different light source. Sketch it out; refine it and try, try again.