Remember to pray for the people and donate as much as possible especially during Ramadan when the rewards for everything is multiplied.
I actually wasn't planning on posting this before Ramadan ends but I realized that it is more important to focus on this in Ramadan due to the extra rewards. Here's a short outline of how people can help:
Contact a local charity organization that is reputable, that you trust, and that is performing charity work in Pakistan. Ask them what is needed and the best means to provide it. In particular, most charities will probably want money as they can direct this to the exact items needed. In some cases, there will be a need for blankets, tents, food, water purification resources, etc. but your best bet is to ask first. Don't assume that your charity can make use of actual items until you are sure.
Volunteer to help directly. If you have the sort of skills that might be useful in Pakistan right now, and you can find an organization to link up with, consider volunteering. Nurses, doctors, carers, logistics operators, builders, transport operators, etc., are all possible jobs you could volunteer in if you have both the expertise and the backing/acceptance of a reputed organization helping in the efforts.
Hold a donation drive either to raise money for the flood victims or collect goods that they need. You could do this at your workplace, school, college, club, etc., and donate all of the money raised on behalf of your group or organization.
Participate in an athletic event to raise money for the flood victims, such as the UN Run for Peace. If you don't want to join an organized event, consider running your own and donating all the funds raised.
Listen to the TV, radio, and check out online news for updates as to what is needed to help out. There will be changing needs as the disaster's consequences unfold and not forgetting about these needs as time goes on is a vital part of being an effective donor.
Contribute to a project that helps to keep people informed about what's going on concerning the flood crisis. Good places to check include sites such as the Pakistan Flood Map on Open Street Maps.