My own "hints"

A lot of these you may already know, but here's some stuff I've figured out in the time I've been playing:

Keeping your silver:

When you are done questing and fighting for the day, before you put your Knight to work and log off, spend all the silver you can. Either use it on upgrading your skills, your gear, buying health potions, donating it, whatever. ** If you have the rubies that you can buy, activate the treasure chest to hide most of your silver if you'd rather not spend it, but they only protect 60% of what you store in them **
The reason being, if you are attacked while you are not on-line and you don't yield a lot of silver, chances are really good that the same Knights won't continue to attack you.
If you give a lot of silver to the attacking Knight, you may become known as a "farm", someone who is an easy target and yeilds a large bounty... NEVER a good thing to be thought of as.

Rubies:

We all know it's a free game, but you can buy rubies from the game makers, the lowest set-up is 100 for about $15 (US). There isn't a huge benefit, especially since this is the only game I've encountered so far where you can win them from people you battle, BUT - one really big help they will give you is that you can cut training costs by 25% or 30 days for (I think) 20 rubies at a time.
So, if you've been mulling it over, that's something that's a pretty big assest.
The only other real benefit of rubies that I've found is that the premium account doubles your quest time, from 120 points to 240 points, which is a big help as the higher levels require more experience points to achieve.

Duels:

Don't attack players at a level lower than you are. They are easier to beat, true, but you will lose experience points by doing it. The more levels below you they are, the more you'll lose.

Missions/Quests:

I'm going to assume this works the same whether you are a "good" Knight or a "bad" one... when you choose a quest, the options with more skulls will lead to higher awards of silver and experience points. Most of the quests are 1 or 2 skulls, but there are 5 or 6 that are 3 skulls and 1 that is a 4 skull.
Also, as you get to higher levels, there will be more quest options that open up (e.g. I think you can choose the Pirate Ship at Level 8...)
Finally, when you choose the longer quest times (20 points = 10 minutes, 40 points = 20 minutes and 60 points = 30 minutes), you stand to win more silver and experience points as well.

Work:

An easy way to pick up a steady supply of silver is to put your Knight to work (using the work button, not the tavern button to be a merc) for the 8 hours you'll be sleeping. In the AM, put him up for another 8 before you head to work or school. By the time you get home in the afternoon you'll have several hundred silver marks banked for about 90 seconds of effort.

I got the pirate ship at level 6 or 7, and the Tree Giants showed up at level 6, I believe.

^ I knew it was once you'd gotten a few levels, I just wasn't sure how many... do you know what level you get the Gaint Spider cave? That's the only 4-skull mission I've found so far.

I have a lvl 12 on the 3rd server, eventually you will get more 4 skulls, every few levels gets new places to quest. Eventually you will have 27 places to quest. I really find that the sea snake and the victory column give the best average silver. Though the spider still seems to give the best gem return, Ive gotten 4 from one mission on the spider.

I've had awesome success fighting the Ents (in the Witch Forest), I get 9 experience points and average 800 silver for a battle with them.