A consolidation of useful tips that are scattered around the site.
Save before every date! This is important if you accidentally say something wrong (like falling asleep during the date o_0) and need to rectify it.
Do not save once you get a bomb, so in case the bomb goes off you can still load and have a while to defuse the bomb.
Check the site once every month and click on all the links (the colour will change when the link has been clicked on) in order to unlock certain date spots and events, some of which are important for getting cutscenes (e.g. Kazuma and the lions in the zoo).
Use the horoscopes to plan your activities for the week, because different horoscope predictions will influence the success rate of the activity.
Though most will wish to stay far away from Goro and all items linked to him, Goro's column is especially useful for you to decide what clothes, accessories and colours to buy and wear for the month. This can increase the relationship state of the more fashion-conscious guys like Madoka and Kei.
Do join whatever club the guy you're going after is in. It helps a lot in increasing your relationship state with the guy, because every time you have a successful club practice, club comp or cook good food during the club camp, his relationship state with you will go up slightly.
A quick way of checking whether there is club practice on the day is to roll your cursor over the friends section. If all the girls except Arisawa (who isn't in a club) are not free (nothing happens when you click), it means you have club practise.
Your first friend (depending on the room you picked) will not be able to ask you along to the the pillow fight in the second year trip (no idea why), so be on good terms with at least one other girl by the trip if you want to go pillow fighting!
One sure-fire way of not getting a girl on rival mode with you is to never play with her after meeting her. That way she will never bother you.
As your first friend, don't pick the girl who will rival you for the guy you're going for. Even though the stats boost in helping you meet the guy is useful, it is not imperative as you will meet the guy soon enough as long as work on the required stat (e.g. read mags for Madoka) early on in the game. You will most likely have to play with her a lot to get your social up to get a job, which makes her more likely to go on rival mode with you.