When I volunteered at a local animal shelter, their guidance for a second dog was "Younger, smaller, opposite sex." Now there are plenty of people who have violated this advice, (myself included) and have happy, well adjusted animals, but it was their stock answer for anyone considering adding a pet.
The most important advice I would give is that the FIRST step is to introduce your current dog to a potential new one on neutral territory, and see how they get along. Don't put your current dog into a situation where you bring in a new animal without their "permission." In fact, any reputable shelter or breeder will INSIST that the dogs meet first to make sure there won't be a problem. |