@Lizzie That's a really good question. Maintaining a "neutral smell" can be tricky.
I try not to use scented products. I use an all-natural cleaner from a local boutique manufacturer that's made with real lemon oil, and the smell dissipates quickly. I also use an air cleaner in the flat. I use unscented laundry products, but I've had people tell me my sheets smell "like a hotel's", so that may be ironing. I also clean a lot, and air out the space thoroughly between guests.
I personally don't like scented products like candles, wax heaters and sprays: they often give me a headache, and they just mask smells.
Hotels and motels use ozone generators. You can buy them for home use if you're serious about wanting to remove odors, especially tough smells like mould and cigarette smell.