Not every player will agree with what I write here, but thusfar, I have the impression that, at least for me, this has been the best approach.

1. Renning:
Before you move past Lv 60, make sure that all your heroes have been renned to the point where you must upgrade assembly beyond Lv 60 before you can ren again. By know you will probably know that the Lv difference between your assembly and the heroes you want to ren, must be between 5 and 9, 9 levels difference giving the highest probability of a major gain when you ren. I don't know what happens if the difference between your assembly and your hero is higher than 9.
During the first hours after a ren, your hero will be weaker than before (but he will catch up later). Here's what I do: during the later hours of autumn (depending on when I get up ;-) ), I initiate a Cease Fire and then start the ren. Spending gold to change the mode of your heroes training and laurels or trans to fast track him is a good idea. If you start CF 8 hours before winter starts, you will have 32 hours that your hero can ren before Zeus or Athena players can attack you again. If that's not enough, you can add another 8 hours as soon as Spring starts, giving you a maximum of 40 hours to bring your hero up to speed.

2. Technology and Formation
Make sure all non-formation technology is at maximum. For the formations that you use the most, make sure they are at maximum as well. You can also upgrade all formations to max, so that you can easily switch between them, depending on the type of enemy you are fighting (e.g. when your enemy has a "Strike in Column" hero in the middle of a "Boar's Head", you may want to use a formation that has only one hero in the middle, and preferably one with good shields and cloaks).
The choice of formation depends on the type of heroes you have. If you rely on BM-heroes, Boarshead seems to be the better choice; if you use more Elemental heroes, Gramarye is the best for them (but remember that Elementals are vulnerable, so you stand a chance of getting wiped out faster).

3. Equipment
Try to have your equipment at maximum level as well. Upgrade shop to the same level as assembly, then upgrade equipment to the highest possible level. This is expensive (and prices keep going up the better your equipment gets). If you want to save on drachs, at least make sure that all your heroes have decent shields and cloaks; that elementals have their books at maximum level (but note that not every elemental needs a book; e.g. Syrinx); that BM-heroes have their mounts at maximum level and since BM-heroes also do regular strikes, their regular weapons should not be ignored.
Don't waste time and drachs on equipment you don't really need. Elementals don't use swords or mounts; BM-heroes are illiterate ;-) and have no use for books; so it is better not to invest in that type of equipment. If you want to increase their faculty, better try giving them horns and upgrade those. In the VIP shop, you can purchase higher type horns.
If you want to give a hero new and better equipment, it is better to upgrade to a higher colour rather than to stay within the same colour. The difference between the lowest grade and the highest grade within the same colour doesn't really justify the change. E.g. at (G7)5the valor shield of yellow shield "Burning Lava" is only 15 points higher than the yellow shield "Caged Desire"; but it is 218 points higher than the green (G7)5 "High Hopes". So swapping a "High Hopes" for a "Caged Desire" or a "Burning Lava" is a good investment, but swapping a "Caged Desire" for a "Burning Lava" is just a waste of time, drachs and effort.

Well, that was my 5 cents of advice, based on how I have experienced the game so far. As I said at the start of this note, other players can and will have different views on this and they will probably work fine as well.

The most important advice I can give, however, is just to keep enjoying the game :-)