My boyfriend has kindly built me a greenhouse (so he does love me after all!!) I planted some tomato seeds around a month ago and put them on my kitchen windowsill (which is very busy with seedlings at the moment), and absolutely nothing happened. Nothing at all. I kept them moist, they got plenty of sunshine. No seedlings. Nothing to go in my new greenhouse

So I have just planted some more seeds - any tips for getting them to germinate? I grew tomatoes from seed last year and was swimming in tomato plants - far more than I could use - but I can't remember what I did differently. It's the same packet of seeds, though, so could it be that they have just got too old? They are still within their 'sow by' date on the packet. I have put the ones I've just planted in the airing cupboard as I thought perhaps they weren't warm enough on the windowsill. Is this a good idea or a bad one!?
Any tips would be greatly appreciated.
PS - I have the same problem with my spring onions. Planted seeds about a month ago, put them on a sunny windowsill. Nothing has happened....