It looks like the forsythia is in need of rejuvenation pruning. This type of pruning is done no more than every three to five years when a shrub begins to look gangly and woody. It works very well on multi-stemmed, twiggy-type shrubs such as lilacs, forsythia, honeysuckle, beautybush, etc.

Rejuvenation pruning should be done in February when the plant is dormant. Take out the oldest, largest and woodiest 1/3 of the branches. Cut as low to the ground as possible with clean cuts.

This pruning will keep the plant healthy, the growth vigorous, and the bloom at a maximum (unless there is a late freeze that kills the flower buds). Note that the forsythia will not bloom the first year it is rejuvenated, but will bloom in future years.

During years where you do not rejuvenate the shrub, continue to prune for shape, thinning, and any dead or diseased wood. These types of pruning should be done right AFTER blooming is finished and before the wood sets bloom for the following spring. Do not wait until fall to prune. Fertilization can also be done at the time of pruning.

Here is more information on pruning shrubs:
http://www.ext.colostate.edu/mg/Gardennotes/616.html