Cards on the table - I'm a major Ikea fan
My tips from my 5 trips in 2 months:
1) Do your research before going. I think the website is more useful than the printed catalogue. Know what you want and make a list. Check the measurements so you know the stuff will fit. It's worth dividing your list into 2-3 categories - Must Haves, Would Really Like to Have But Can Wait For and Might Get But Not Sure. Then you can put your energies into getting the Must Haves and see how shopped out you are after that. I think Ikea is great for storage furniture in general (shelves, cupboards etc), lighting, kitchenware, I could go on and on

Be careful if buying beds, mattresses or bed linen. I think they are great quality and value but they are continental sizes, not the British sizes we are used to.
2) Check the really important stuff is in stock (online stock checker or ring the store).
3) Have all your measurements to hand for any impulse buys.
4) Choose your travelling companions carefully. Leave the children at home if possible - although it can be a nice family day out, I reckon with the long journey it might be a bit much.
5) Wear comfortable shoes!
6) Go as early as you can. Weekdays are best.
7) Resolve beforehand that any arguments/tiffs in Ikea stay in Ikea.
8) Before picking up the items in the Market Hall, do the Self Serve/Full Serve items. That way you won't run the risk of them being out of stock by the time you get there and you won't have to deal with your trolley full of kitchen gadgets etc whilst trying to heave huge flatpacks around. Bear in mind there can be delays waiting for Full Serve items so the sooner you can deal with that the better. For Full Serve, the floor staff print you a sheet with a bar code which you take to the till and pay for. Only then is the item reserved for you. You hand in the sheet of paper and receipt at the Full Serve desk and they go and pick the item(s) for you. Meanwhile you can always go and do more shopping while waiting.
9) Take plenty of breaks (free tea and coffee on weekdays) and make sure to try the Daim cake ... mmmm!
10) If any Self Serve items were missing when you first checked, have a quick look before you leave. I've sometimes been lucky enough to find they are back in stock after being returned.
11) Leave before you totally run out of steam. You can always come back again (as I found to my credit card's detriment).
12) Stock up at the food shop before going. I love the reindeer salami, mini Daim bars, sour cream crisps and Skum Mushroom sweets.
13) Have a hotdog for the road.
14) If you have the energy, stop by Sainsbury's down the road to pick up some wine. Great value while sterling is so cheap.