Observing globulars is an art. The clusters are small and tightly packed. Most require at least an 8 inch telescope to resolve any individual stars, but even these can have interesting shapes and textures in small scopes. Don't be afraid to use a range of magnifications in order to get as complete a view of the cluster as possible. If you are trying to resolve a cluster, hold your eye still for at least 30 seconds while looking off to the side of the eyepiece field (averted vision). If you are lucky, you will start to see the cluster stars begin to pop into view like little pinpoints of stardust.
The brighter globulars are visible in binoculars or even to the naked eye, but because of their small size are difficult to distinguish from faint stars. Your best bet is to go with higher magnification (10x50 binoculars work well). Some noteworthy globulars besides M13 are M22 (brighter than M13 and resolved in a 3 inch telescope, but far to the south), M4 in Scorpius, M92 in Hercules, M15 in Pegasus, M2 in Aquarius, M5 in Serpens, M10 and M12 in Ophiuchus, and M3 in Canes Venatici. See the database and my article on Overlooked Globulars for these and more.
CCD Images of Two Globular Clusters