Personally I try to avoid painting yellow because its one of them awkward colours that takes several coats. But thats not what you want want to hear, so heres some tips i've picked up over my years of painting.
Depending what you are painting, use a white basecoat. Yellow likes a white basecoat.
Take your time and put on several layers. This can be annoying if you have lots to paint.
My prefered method is to use a Zamesi Desert Base coat. Zamesi Desert paints over black nicely and sunburst yellow paints over Zamesi nicely.
Hope this helps.