The most commonly used overland crossing from Thailand into Cambodia is at Aranyaprathet, which lies just across the border from the Cambodian town of Poipet. The border is open daily from 7am to 8pm and officials will issue thirty-day Cambodian visas on arrival (see Thai Visa www.thaivisa.com for its visa-run forum, and Tales of Asia www.talesofasia.com/cambodia-overland.htm for a very detailed description of the crossing). Once you're through, it's about three hours in a share-taxi or tourist bus to reach Siem Reap, 150km away.
If you need a hotel in Aranyaprathet, try either the comfortable fan and air-con rooms at Inter Hotel at 108/7 Thanon Chatasingh (037 231291, www.ourweb.info/interhotel ; fan Price: B250-400, air-con Price: B400-600), or the cheaper en-suite rooms at Aran Garden II, 110 Thanon Rat Uthit (fan Price: under B250, air-con Price: B250-400).
Travelling to Aranyaprathet from east-coast towns, the easiest route is to take a bus from Chanthaburi (which has good connections with Trat, Rayong and Pattaya) to the town of Sa Kaew, 130km to the northeast, and then change to one of the frequent buses for the 55-kilometre ride east to Aranyaprathet. From Bangkok, you can travel to Aranyaprathet by train (2 daily; 6hr), though you'll need to catch the 5.55am to ensure reaching the border before 5pm; the other leaves at 1.05pm. Return trains depart Aranyaprathet at 6.35am and 1.35pm. Tuk-tuk taxis will take you the 4km from the train station to the border post. Alternatively, take a bus from Bangkok's Northern (Mo Chit) Bus Terminal to Aranyaprathet (at least hourly; 4hr 30min), then a tuk-tuk from the bus station to the border. The last Aranyaprathet– Bangkok bus leaves at 7pm. It's also possible to buy a through ticket to Siem Reap from Trat and Ko Chang, or from Thanon Khao San in Bangkok (B200–600), but this option is dogged by scams (including a visa "service charge"), can take up to 10 hours longer than doing it independently, often travels via the less convenient Pailin or O'Smach border crossings instead, and nearly always uses clapped-out buses or even pick-ups on the Cambodian side, despite the promised "luxury bus".