You have a couple options, and those options are dependent upon the scenario. Ultimately, in a defensive situation they are going to be more effective. Offensively, they're a bit harder to use.
Defensively you have a couple choices. Personally, I prefer to use some dug-in field guns with body armor. Their cheap and effective. I'm a fan of the Light Gauss, since you get 2 LGRs and 16 shots to start the game off. They're good on a flank for protection or on high ground for sniping. UAC 10s or AC 10s w/ precision ammo are also very effective. Another option, and this is really only effective if you are given the whole map as a starting zone, is placing your infantry out of LOS behind hills or deep in forests (preferably hidden). These units will effectively negate useful terrain features for your enemy, as they will end up getting bogged down by the infantry if they attempt to use the terrain to their advantage.
Offensively your choices are a bit more limited. They aren't extremely useful in terrain-less maps, but become increasingly more useful the more terrain there is. When paired with speedy APCs, you can effectively use the same strategy I suggested above for terrain denial...it just takes a turn or two to position them.
Conventional infantry tends to be very similar to Thunder LRM mines. They're more effective as a psychological weapon than as an actual weapon in game. But their low cost means that even if they never fire, the impact they have is nearly always with the couple hundred BVs you spend. Additionally, use them in situations where you have good intel to suggest you're going up against the typical "Lyran Wall of Steel" who might not have good answers to conventional infantry which can become good tarpits for their big guns, forcing them to kill one guy per gauss or PPC shot.
And as you mentioned, they shine in urban settings for sure. Hope some of those ideas helped. Really, the key is to start throwing a couple units of infantry into the mix in the games you play. Once you play with them a bit, you'll start to see how they can be effective and ineffective and learn from those experiences.