Please read these two excellent PSW posts by Raikage. I don't enforce RP as strictly as PSWIII did but talking IC as much as possible and doing consistent RP is required!
Roleplaying
Now that we are not too far away from completing the characters in terms of mechanics, I'd now like to talk about roleplaying. Roleplaying is supposed to be a big part of the Philosopher's Stone War, although it was somewhat neglected in PSW II.
Firstly, I need each Master and Champion's personality, history and appearance. This should detail how your Master learned magic as well as his motivation for taking part in the War. Notice that I ask you for your characters' alignments. Your personality should match your alignment, and this actually does have an effect. If you're supposed to be a Lawful Good guy, I won't let you go around rampaging and killing civilians. It just plain doesn't make sense. Similarly, if your history involves you being a psychotic killer with no self-restraint, I won't allow you to interact too calmly with civilians, so keep that in mind. I know some players might be tempted to choose their personality for maximum benefits, but I urge these players not to do so. Roleplaying is half the fun of the game!
Aside from motivations, you should also consider how and in what circumstances your Master summoned your Champion. Perhaps your Master retrieved a suitable relic and used it to summon the Champion he was looking for. Perhaps your Master was in grave danger, and inadvertently summoned the Champion to save himself. Try to be creative.
Something to consider is also your Master and Champion's interactions. Your characters are a duo and they will interact with each other more often than any other characters. Your Master and Champion do not necessarily have to have similar alignments. It's possible to have a good Master control an evil Champion, bending the champion to his will. Of course, the Champion is unlikely to be happy with this! Instead of viewing this as a drawback, look upon it as an opportunity. This is a great way to introduce some character development.
I noticed that many players like to do other things when it's not their turn in an encounter, be it talking to other people, reading manga, or other distractions. I understand if a player has restrictions (like doing homework or staying up till 5 AM in the morning to get an encounter done). If that is not the case, however, I urge players to use the time when it's not their turn as well. Thinking about your next move at the very least speeds up the process.
What I really want to bring up, however, is that roleplaying should be a concern as well when you're in such situations. Speak in character when you can, and develop relationships with other characters. For example, it is much more fun and believable if two Masters constantly target each other due to conflicting beliefs, rather than 'I need to attack the other guy cause I can't win the game otherwise'. Perhaps one Master is a young idealist, whereas the other is a jaded old man. Maybe one Master is just an outright villain, and the other believes strongly in justice. Such interactions are possible with Champions, too. A Christian-hating Champion would have a field day in a battle with a Catholic Saint, for example. Even if you don't encounter a perfect fit, it's possible to develop rivalries or respect for each other.
The metagame has changed in this game due to the Master-->Summoner change but these points still are very valid.PSW Metagame
Over the course of 2.5 Wars, a metagame has been built up and I think it's only fair to warn new players about it!
Protect your master!
Your master may have a lot of HP (even more than Champions at times!), but s/he also has the armour and saves of wet tissue. A full attack or high level spell from a Champion can very easily devastate your Master, so always be wary of this danger.
Teleport/Dimension Door VS Dimensional Anchor/Teleport Tactician/Rider's Chase
This is a very common feature of the PSW metagame. People will usually attempt to run away from a fight when they're losing via teleport or DD. Standard countermeasures have developed, usually involving Dimensional Anchor since any Master can cast it. Some champs may take Teleport Tactician to make it harder. And of course, Rider can chase across teleports.
Alliances
Alliances have grown to be a common feature in PSW. I can't exactly outright ban alliances with more than two people but I very heavily encourage players not to do so. On the flip side, those who do not plan to ally with someone should probably come up with countermeasures for situations where they are outnumbered.
Spell selection
There's a very wide variety of spells available to Masters. Some spells that don't seem that useful can prove their worth against certain opponents or to counter certain other spells, so keep an open mind. Also, I would advise players against over-relying on one spell.