I've worked in and around nonprofits my entire career. When creating a charitable giving strategy, I always recommend 2 things:
1) Give monthly if at all possible. As mentioned above, this really helps with budgeting. If you can't give monthly, try to find out what the slow season is for your preferred org. Most orgs get many donations around the end of the year. Some have annual fundraisers that provide a big influx of cash at a specific time. They all have a slow season when a lump sum could be really helpful. In my case, our local cat rescue really needs donations in the summer, so I try to give them a good sum then.
2) When choosing a charity, try to visit the org personally. Many orgs will be happy to do a donor tour. If they can't because of sensitivity (domestic violence shelters come to mind), they will have someone available to talk with you and answer any questions. If that is not possible for you, Charity Navigator is a decent financial evaluation site.