We make up our LPS fresh on a daily basis. We found that solubilising the lyophilised LPS at day one and then storing it increased the variation between experiments from week to week. We weighed out the LPS daily, made up our dilutions and used them on the day. By doing this we reduced the variation between experiments.
You can prepare 1 mg/ml stock in PBS containing 0.05% tween-20. This can be alliquoted and stored in -20C. You can use this for atleast 1 year. Weighing will cause more variations as the amount weight will be very less (1-5 mg).
Ideally, you purchase 5mg LPS (we usually use B055 strain LPS). Reconstitute and store in -20C. the inflammatory activity and neutrophil phagocyte activity remains similar always