From Europe by rail, take the Eurotunnel, then go from Waterloo to Paddington using the Underground, and from Paddington to Reading or to Henley direct. During the Royal Regatta the trains to Henley are increased in number and timed to fit the needs of London spectators (more or less!). The train station in Henley is not far from the river, a five-minute walk to the Bridge or closer if you want to detour to Marsh Lock or the River and Rowing Museum.
Those going by road (and trailing), the route depends on your direction of origin. From London or the Continent, use the M25 until you reach Junction 15, and take the M4 westbound to junction 9a. Turn right onto the A423(M) and go past the turnings to White Waltham and Maidenhead until it reverts into a non-motorway dual carriage road. Just after this point is the turn to Henley, and you then follow the A4130, a pretty winding but busy road, straight to Henley, coming down Remenham Hill to the bridge into town.
From the south, go to the M4 and follow to junction 9a, then as before. From the north, the M40 is the best route, down to junction 4 (High Wycombe turn). Take the Marlow road here (A404), and go down this dual carriageway past the Marlow and various village junctions. Straight across the roundabout at Bisham Abbey, onto another dual carriageway stretch, and then turn off at the next exit, onto the A4130 to Henley (as above).
From Henley Bridge, the Henley Rowing Club is on the road to Wargrave, a slanted turning at the foot of Remenham Hill. It is only a few hundred metres along here, on the right before the road goes up the next hill. Leander Club is right next to Henley Bridge, on the downstream southern side, with a turn-off into Leander Way, right opposite a new building constructed a few years ago for the Royal Regatta Headquarters. To get to Upper Thames Rowing Club, you go fifty metres further along the main road from here, with the bridge behind you, and turn left into Remenham Lane. Upper Thames is 700-odd metres along here, after a Z-bend and just before the lane turns up a steepish small hill. Those visiting during the Royal Regatta please note that a) parking at all these clubs is heavily restricted and b) travel along Remenham Lane is strictly controlled, switching direction from morning to evening traffic.