I notice many servers tend to have many kernels, alot of the time people do not clean these up, this can obviously lead to /boot becoming full, there are a few ways that the unused kernels can be removed, Remember you should be running the latest kernel when possible
The suggested method would be to use package-cleanup which is part of the yum-utils package.
yum install yum-utils
Once yum-utils is installed you can clean the unused kernels with the following command
package-cleanup --oldkernels --count=2
This command would keep 2 kernel versions, you can obviously change this to however many you would prefer to keep.
Another method which I would not suggest is to use something like the following to compare the running kernel against a rpm list and then remove the ones not in use.
rpm -qa | grep kernel | grep -v $(uname -r) | xargs -i yum remove -y {}