S-Link Advice:
1) When given the choice between developing an S-Link and building up your stats, ALWAYS choose to work on your S-Links. I never spent time raising stats unless there were no S-Links I could do instead. The only exception is around exams, when it can be worth it to build up your knowledge.
2) When developing S-Links, always try to have a persona of the same type. It makes a huge difference in how quickly you can progress through the link. It's totally worth it to buy a low-level persona out of the compendium, just for the express purpose of speeding up that S-Link.
3) Try to prioritize S-Links with party members, or ones that build stats (Sun, Strength, Temper., Devil, Tower). The others can wait.
Oh, and minor advice, not really S-Link related. On nights where you can't do anything else (because you're tired from dungeoneering, or you're supposed to watch the midnight channel), CHECK THE FRIDGE. You can't do anything else, but you can still get that courage bonus sometimes at least.