If you MUST use an online wallet for a significant amount of time, then do the following:

1. Only store small amounts of BTC on an online, web wallet. This way, if it gets hacked (even if the server gets hacked), then you only lose a little bit.

2. I would take the highest security measure and use a $200 notebook that only runs on a bootable USB drive (clean OS install each time) to manage your online wallet. Never use that notebook for anything else but logging onto your online wallet and conducting transactions. This way, there's a very infinitesimal chance that your notebook will ever get malware that could steal your online wallet's login info.