You can trade cash to the other Covenants. Trade equal amounts (for instance, trade them 3 gold for 3 gold). Click on your gold, set the amount to be traded, then click on their gold and set it to the same amount. This will increase your relations with the other Covenant without losing influence or crystals.
When you face a quest Boss you have no chance of defeating, restart the game with other Covenants set to a higher level than your own (preferably as high a level as you can use and still use the relationship trick). Once done, trade equal amount of gold with a Covenant until your relationship is high enough to start an alliance. Then, select "Adventure Together". Go to the quest Boss and stay back while your over-leveled Covenant partner kills it. Next, restart the game at the normal difficulty setting. When you get to the same quest, you can select "Skip" and continue with the next quest.
-Before you sell, enter the "Relations" screen and go by each Covenant. Ask if they want anything. If they want something, try selling it to them at double selling price or half the retail price. If they respond yellow, lower it by ones until it is green. Once done, sell everything else. Never buy potions, as they are nearly useless. Always use food.
-After defeating the final Boss, return to your hometown. When it is dark outside, go to the crystals outside your home. Turn the two orange crystals off, then turn the two red crystal on. Click the center blue crystal to open a secret passage which normally leads to four small gold chests. Collect all the items out of these chests. To collect them again, save the game and exit to the main screen. Click "Resume Last Game", then repeat the process until it is day.