Eyes are the windows to the soul as the saying goes. We are drawn to eyes to determine intent and reaction. We all instinctively scan others for imperfections so that when the eyes in a portrait are not accurate or soulful we see the mistake immediately. I sometimes spend as much time on an eye as would for hair for the reasons 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.
Look at shapes, subtle curves, overlaps.
Work with a magnifying light; get in close.
Spend the time; don't settle for good-enough.
A couple of tips from this piece:
First, 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, a reminder about 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. The common reaction is to move onto something else. But before doing that we need to challenge ourselves to try again. There are always alternatives ways to compose to a painting. Think about what you'd see if you asked 10 artists to each paint a scene - you'd get 10 completely different interpretations!
Try this - explore several alternative compositions by doing a series of thumb-nail sketches. Come up with a different plan - a new color palette or a different light source. Sketch it out; refine it and try, try again.