Walk on Regent Road (in the west of the city) until you get to Salford. There is a big petrol station with many slip roads, which might look attractive, but the smaller petrol station (on the left side of the road) offers a more convenient spot for drivers to stop, and also allows drivers that don't enter the station to pull over. This is a link to the spot
Alternatively, you could keep on walking along Regent Road until you get to the "roundabout" and get to the shoulder on M602. Getting there is a little complicated since it's not very pedestrian-friendly. Technically, you'll be on the motorway, but it's right behind a traffic light so traffic goes slow - a police car passed by uncle_sam01 without batting an eye. Make sure to have a sign (with a motorway number), since it's not clear which direction you could be going. Link to Streetview
Take bus 167 from Piccadilly Gardens, Lever Street/Dale Street (Stop E) in central Manchester, and get off at stop: Birch, Heywood Old Road/Langley Lane (journey time approx. 25 minutes). The service is hourly (more info on times from: (http://www.traveline.org.uk/index.htm). You should be able to see the services when approaching the bus stop. When you get off the bus, walk down Whittle Lane (which goes over the motorway M62), and you will be at the services, where you can hitch a ride down the M62 to Leeds or Hull. The bus costs around £2.50. You can get cheap coach tickets from Megabus or National Express funfare (they have websites) where you can get trips from between £3-6 if you book in advance. So I would only take this hitching route if you are on a low budget or traveling last minute.
Update (March 10, 2016): Bus 167 is no longer in service, however bus 163 can take you pretty close to the services mentioned above. The fare from Picadilly is £3.80 though, so doesn't much sense. Another option is to walk down the A56 (Bury New Road) and wait at the exit ramp of the ring road merging with the M60. Alternatively you can take the tram to Prestwich to get there.
South Towards Birmingham and Bristol
1) Catch a bus to Whythenshaw (the 101, 104 or 105 from Piccadilly Gardens in the city centre. Just ask the driver to shout for you at the Wilbraham road junction if you're unsure where to get off) and get to Princess Parkway, this is the start of the M56. From here you can head to the M6, better if you are heading south since you are a few junctions south then if you was on the M602. here you will see a petrol staton immediatley befor the slip road, I've always had a quick ride from here, mainly for drivers filling their cars up, and a bus stop in front of you for cars to pull over safely. EDIT: Dec 2016. This seems to refer to the Texaco gas station pretty close to the sliproad to the motorway. When I approached drivers there I was sent away by a member of staff
2) You don't necessarily need to be on the M56. There are a few good spots on the A5103 (Princess Parkway) so depending on where you start from in Manchester you could head to any one of these. I walked down the Wilbraham Road heading west and turned left (south) onto the A5103. 50m further is a Shell petrol station with fairly good flow. Although some of this will be local traffic, a lot is also continuing onto the M56 and then M6. If you don't want to wait around at the garage itself, there is a good bus stop just before so you could try some road hitching here. It took 10 minutes getting a ride out to the M6. EDIT: Dec 2016: There is no Shell petrol station close to this spot (anymore?), at least I couldn't find one.
3) There is this bus stop . It really is a great place to get south from; I've used it six times and have never had to wait more than 1 hour 15 - but it usually takes much less. One time it only took 90 seconds to get a lift as far as Birmingham.
4) After trying unsuccesfully to get a ride from the positions mentioned above, I walked to a bus Stop at the A560. It is located between two roundabouts, a bit before the roundabout leading on to the M56. There is an exit for an industrial estate in front of the bus stop whitch a decent amount of space for drivers to slow down and drive over, where I stayed to wait for a driver. Only took me 5 minutes to get a ride all the way to Birmingham. The Buses 104 and 11A can get you all the way there. Position on Google Maps: .
It's useful to know that if someone isn't actually going on to the M6 but carrying on to Chester or somewhere further west then it can still be good for you. There is a big service station about 1 mile after the junction between the A556 (Chester Road) and the M6. It's called "Knutsford Services". So even if the driver drops you at the junction you could probably stealthily walk the distance or just ask the driver to take you there seeing as it's so close.
If you don't want to hitch out of the city, a train to Knutsford costs around £6 and the walk from the train station to the services is around 30 mins.
Hitchhikers who live here
Tom lives here. Come say hi.
Many University of Manchester buildings have shower facilities for students and staff who commute by bike. Some buildings have access gates which require you to present a student ID - if this is the case say you forgot your wallet at home and the reception staff will ask you to sign in (it's just a piece of paper, for student no. make up any 7-digit number that begins with the no. 7). Here's a map of buildings with shower facilities and access restrictions. (log in to Google required)