A couple things that we have used:
1. Utilize part-time staffing to cover your evening hours. You can use contract or agency employees to fill this void.
2. Overstaff and then "shed." Make sure you have enough or even more than enough people to handle the volumes and then should volumes fall lower than forecast you can offer employees to have time off without pay, utilize their vacation/comp time, etc. You would be surprised how many people will actually take advantage of the time off without pay.
3. Use call routing to utilize agents in other (perhaps specialty) skills that are not as busy. In our Avaya PBX this is called Reserve Level routing. You can set certain thresholds such that a call will only be presented to those agents if the queue reaches a certain threshold. For example: say you have Bilingual agents that primarily handle Spanish-language calls. You can add the English calls to their skill set at a Reserve level such that they will only receive an English call if the queue reaches X number of seconds. This is part of the Avaya Business Advocate package and is also called Service Level Supervisor. You can read more about it at
http://www1.avaya.com/enterprise/brochures/gcc0467.pdf.