No hard and fast rules Sponsors generally need to be well known, produce something tangible you are likely to use or just be associated with a club for a long time to become 'iconc'.
SUFC and Laver, Arsenal JVC, Man Utd Sharp (or even Sharp Viewcam) Seville Super Nintendo, Fiorentina 7up, Coventry Peugeot, any beer sponsor - all good.
Online betting companies, payday lenders etc - bad.
In no order other than me remembering them:
Westfield Health - Health insurance doesnt exactly set the pulse racing but this was a good sponsor given the subtle logo and the fact it was a well known local company. Wasn't a fan of Gilders VW on the away kit, sponsor was big and out of place on the mustard colour shirt.
Visit Malta - bit unusual at first but it wins points for a decent logo and the fact they stuck around for 3 seasons. Also came with a mandatory set of underwhelming pre-season fixtures.
Topspring, SA Buzz, Vr Sports - did they even exist in real life? I'd lump Midas Games in with them but their logo gave us that infamous away kit. Anonymous and instantly forgettable
Redtooth - similar as above but they gain a few points as I've seen their quiz sheets in pubs.
Hfs Loans - awful logo and a pretty dull financial service (not even a proper bank/building society)
Sheffield Children's Hospital - think we had these for one game against the Pigs. A worthy cause obviously but the yellow and blue logo didn't look right on our shirt.
Capital One - again, very boring, VISA, Mastercard or AmEx would have been a different story. Also looked horrific in a funny little box on the shirt. Had already been used by Forest as well (didn't we once play them in a pre-season friendly called the 'Capital One Cup?')
Desun - Chinese cordial manufacturers, never seen in any in the shops the shops.
Blades - Horrendous logo ruined what could have been two of our best ever kits. Should have just gone without. Was bad enough at the time we were in that much disarray we couldn't even arrange a sponsor after Ward's disappeared
Teletext Holidays - should lose points for being an online platform but I have to say this was a great sponsor. An improvement on Alpharooms which, whilst not being a bad sponsor, lacked the je ne sais quoi of Teletext.
John Holland - An ugly logo stuck on the first Adidas shirt as a complete after thought ruined it. Much improved in the 2nd season.
Ramsdens - ugly sponsor out of sync with the colourscheme, pay day lender and more readily associated with Middlesborough. Only saving grace is they have some High Street outlets. Despite all this it just about worked, probably because will now forever be associated with one of our most fondly remembered seasons.