There's the standard value/event/boost in your game. Let's say it is +3. So you have a sword that gives you +3 to attacks. Maybe ok for one item, but how do you make your items less boring as an overall set? And add strategy to the game as well? Here's some ideas:
The Trade Off
+6 but you have to take 3 damage to use it. It makes you ask questions: "Can I get healed easily?" "Will this damage put me too close to the edge?" and so forth. Adding interest. Would you rather just have the +3 basic sword? Sometimes you would. Sometimes is a good answer. It means you have to evaluate some states of the game to be effective. And make choices.
The Boost
+3 and +1 for each energy you spend. See what I did? It also gives you the same numerical value, but it adds in a question, which is "Is this a good/efficient use of my energy right now?" Therefore, more strategically complex.
The Permanent Boost
+3, but if you are level 3, +6. It transitions into a better item as other things progress. Similar to above, but there is more of a "one time cost" or "long term payment".The Situational Boost
+3, but +6 in swamps. If swamps are super rare, maybe its +12 in swamps! This is one of my favorite types of changes to make, since it encourages people to change their play patterns if they have an item or power like this. They have to play to it. They have to set it up. There are a ton of ways to implement this, depending on your game. +6 vs Dragons, +5 on cloudy days, and so forth.
The Situational Combo Penalty
Restricted uses
Once per round, once per turn, once per game. You get the idea. I realized the other day that "once per game" is pretty much the same as "discard to use". So: "Discard for +9 in an attack." Powerful, but you have to decide when is best to use it, which makes your decision making more interesting.
More...
The more complex your game, the more complex these can be. If there are different things to accumulate, like Gold, Experience and Energy, you can make different items boost by paying different costs.
And you can make up many others if you have in mind a general idea of relative worth of different things. How much gold is one point of attack value worth? How many XP equals 3 Gold? This is a generally good idea to figure out anyway because it helps you avoid underpowering and overpowering game elements.