The question posed by Joshua when he addressed the Israelites is a bit of a false one. "Choose for yourselves this day who you will serve, whether your God....or the gods of the people whose land you are about to enter." Yes, this is, in the grand scheme of things, the real choice. We will either serve our God or we will serve some other god (or something we have set up as a god), but in the practical, lived-out nature of service, it's not so simple.
For example, sometimes when you are serving your God, you are also serving your brothers and sisters. This was the case for Joseph. He was dutiful in his service to the Lord, but in the end of the story, we find that his service to the Lord was not only for the Lord's glory, but also for his brothers' preservation. Interesting that this was the man who was hated by his brothers for his dreams that they would one day bow down before him. They did, but only because he was the only slave who could save them. Funny how that works, huh?
That's not to say that every time you are serving your brothers and sisters, you are serving your God, however. There are plenty of things that we do for one another that are not particularly God-pleasing. But it is often true that what we do for God is of benefit to our brothers and sisters, so pay attention to how God is weaving these threads.
On the other hand, not most of us would say that we would choose to serve gods other than the Lord our God, but we do it in practical terms all the time. We wouldn't say that we serve money, but we do. We wouldn't say that we serve ourselves but we do. We put these things above all other things, and they become our gods, whether we intended for them to be or not.
But neither does it follow that every time we make good use of money, it replaces our God. Or even that when we do something for ourselves, that this is idol worship. There are some very good, God-honoring ways to use money and to care for ourselves. These are called stewardship and discipline, and they do not need to be wicked. They can be very good.
So it's not as simple as saying, "Pick your god." What matters is how we approach the things that either are God's or can become gods in our lives. And we have to know how it is that we're approaching these things.
The question, then, is not "What are you doing?" but rather, "Why are you doing what you are doing?" Are you doing it for the glory of the Lord, out of the recognition that you were created for this very purpose? Or are you doing it for some other reason? Any other reason than the glory of the Lord is idolatry. But both idolatry and glorification are slavery. They're service.
So you're a slave, of course, as you knew that you are. But a slave to what? And why?