How can I take care of myself while trying to help someone with an addiction?

First of all keep in mind there's a difference between actually helping someone with an addiction and enabling someone with an addiction, if you're helping them to stay clean and stay away from the substance with positive reinforcement that's great - but if you're doing what you can while they're still doing the substance and you help them with money, etc to support their habit then thats enabling. Try to first realize which one of those is happening and if you're enabling then find ways that'll stop that such as not lending them money or items, not paying for anything for them besides maybe food and stuff. How long have they had the addiction anyways and how bad is it? May be time for some sort of intervention or putting your foot down and taking the power. But don't lose yourself in the process, if its starting to bring you down, make you miserable, ruin your home, relationships etc, then you need to remove yourself from that person or do one of the options I said above, because at that point you're not helping either of you.

Tell them to try to limit themselves to fewer times a week/day. Or to only spend this much money for the addiction and nothing more.