In addition to the previous suggestions:
With very small gels, the sample composition and depth (in the well) have a strong effect on the resolution. Rinse the wells with TBE buffer just before loading, as urea from the gel diffuses into the well and may prevent the sample from settling at the bottom. Minimize the amount of salt in the samples; try to load very small volumes (1-3 ul), even if this means longer exposures later.
