Attacking portals equipped with mods(shields, force amplifiers, and turrets) can be a challenging thing. Here are a few strategies that some players use when laying siege to these types of portals.

On Empty Strategy
With this strategy, you purposefully use up all of your XM. Then slowly approach the portal, collecting XM. Once you have enough to fire your Xmp Burster, let it loose, depleting you back to 0 XM. When you get zapped, there is nothing to be taken away. Rinse and repeat until your target has been crippled.
A Mile And a Day Strategy
With this strategy, you take advantage of the portal mod's limited range and attack far enough away so that you do not get zapped. To know how far away you need to stand, you multiply the Xmp Burster's level that you are using by 10m. E.g. if you are using a L7 burster, you need to stay 70m away. The goal is to cripple the portal defense mods without taking a penalty. This strategy may take slightly longer than the On Empty Strategy, but is better for those who cannot burn off their XM ahead of time or just don't like getting zapped. :)