By Bus & Boat -- There are still no direct buses to Monterrico from Guatemala City, although with the new bridge, I expect that may change. To get here you can take any bus from the main Zona 4 bus terminal heading to the El Salvador border, and get off in Taxisco. Several lines service this route, and buses leave at least every half-hour between 5am and 9pm. In Taxisco you can transfer to a local bus or taxi to La Avillana, where you'll pick up the ferry to Monterrico. Buses from Taxisco to La Avillana are usually waiting and leave roughly every hour. Ferries run almost constantly during daylight hours between La Avillana and Monterrico, and tend to leave whenever full. The entire fare, including the ferry, should run you around Q30 ($4/£2), slightly more if you take a cab from Taxisco to La Avillana.
From Guatemala City, Antigua, or Panajachel, any tour agency or hotel in town can book you a minibus shuttle for around Q150 to Q225 ($20-$30/£10-£15). Given the convenience, I think it's worth the splurge.
By Car -- If you're coming by car, you can take one of two routes. In either case, you'll head south out of Guatemala City on CA-9 toward Escuintla. The most direct route is to drive from Escuintla over to Taxisco and then down to La Avillana. In La Avillana, a car ferry should be waiting to take you across to Monterrico. The 20-minute ferry ride costs around Q60 ($8/£4) each way, and tends to run during daylight hours, with the occasional cushion of an hour or so on either end. These leave according to demand.
The alternate, and more dependable, route is to head straight from Escuintla to Puerto Quetzal, and then on to Iztapa. At the end of the road in Iztapa, you'll cross the new bridge to Pueblo Viejo, where the road picks up again for Monterrico. From Pueblo Viejo it's 26km (16 miles) on a well-paved road to Monterrico. There's a Q15 ($2/£1) toll each way on this bridge.
Monterrico is small enough that you can easily walk anywhere in town. There are no official taxi companies in town, but your hotel will probably be able to find someone who works as a freelance taxi driver if you need one.
Both routes into Monterrico will land you on the Calle Principal, which runs perpendicular to, and dead-ends at, the beach. Most of the hotels are located on dirt roads running parallel to the beach in each direction off of Calle Principal. There are a host of cut-rate budget hotels and simple comedores all along Calle Principal.
Fast Facts -- There are no banks, ATMs, hospitals, or major services in Monterrico. Be sure to bring as much cash as you think you'll need, as very few establishments here accept credit cards. There is an Internet cafe on Calle Principal.