Integrating Weight exercises into my Training Program

One piece of training that I am still not sure how to integrate into my workout schedule is weight lifting. Often I feel I should treat the lifting program as a separate program running in parallel with the endurance (swim, bike, run) training that I do. However, no matter how hard I try to do this, I always find myself thinking about the next endurance workout and worrying that my lift will negatively affect it. For example, lifting back and biceps the day before a swim routine would certainly affect the swim. Honestly, I don't know what kind of effect this would have in the long run (no pun intended). The result of my uncertainty has resulted in a decreased number of weight lifting sessions.

My current weights program is not well defined; I will admit that. However, I think I am forming a good idea of what to do going forward. I have decided to do what I will call half body workouts (upper body and lower body). The reason I have split it into these two groups has a few reasons. First, it will allow me to merge the workouts into my training schedule based on what endurance event I am training that particular day. Second, they are both very very wide categories and I can rotate in and out a large number of exercises. I'm hoping this will prevent the urge to work the same muscle groups each time.

The Plan moving forward: Exercise with weights two days per week. If I am planning to run or bike that day than I will lift my upper body. If I am planning to swim that day, I'll do a lower body workout. Finally, I will be doing one of each each week. Again, these are just guidlines I am going to try to follow. I am slowly trying to build into following a set plan, but for now I am sticking with the general guideline approach.

Anyone out there have a good weight program integrated into their Triathlon or endurance training programs??