The majority of cell phones today use lithium-ion batteries that cannot be overcharged according to Gizmodo. Additionally, smart chargers prevent batteries from overheating, which can reduce their life.
Cell phones previously used nickel-based batteries, which did suffer from the "memory effect." Batteries had to be fully charged from empty or the battery would forget its capacity. That is no longer an issue with lithium-ion batteries.
Technology experts suggest topping off the lithium-ion cell phone battery often. Maintaining a 50 percent or higher charge is ideal. Allowing a battery to run down to empty before plugging in for a full charge is recommended once a month but should not occur regularly. Keeping batteries cool is also important, as excessive heat will cause these batteries to age drastically.