Taking good photographs of your baby requires many things but above all patience! Your tiny subject is completely unaware of posing, lighting, shadow, glare or any other aspect of portrait photography. Put a good amount of time aside to get the shot you want.
Very small babies can only be left safely when laid down, posing isn't really an option. Surround baby with supportive pillows and cushions, may be a bean bag, and never leave them on their own. Bean bags are great for supporting the little one into a fully supported position as they are surrounded by the bag instead of being propped against it as they would be with blankets and so on. You may be able to seat them in a slightly more upright position, but if too young experiment with your own positioning instead.
Try new angles from which to take your photographs and have hand or desk lamps around which can easily be moved. Be sure not to have any lighting too close to baby as they can get very warm and are very bright close up. Try not to use a flash as this can be a little scary for someone too young to know what it is. To try to get their attention you may want Mum or Dad to be behind you calling to the child in order to get them to face the camera. This is not guaranteed to work! Toys out of camera shot may also work, or even in shot if you want to use them as a prop. Position yourself at eye level with your little subject, this gives better perspective and more focus on your little subject.
With slightly older babies they may be able to sit up but as much as this aids your chances of getting 'the' shot, the fact they can move around more greatly lessens your chances of them sitting still! There's so much for a baby to see and do at this age the chances are as soon as you pose them, they're off again! A faster shutter speed will increase your chances, but as before, patience is key. A tripod will also help, limiting the chances of 'shutter shake' and you taking a blurry picture.
Play with backdrops and props, with a nicely arranged backdrop a sleeping baby photograph can create an adorable photo canvas print. Of course you must be sure at all times that baby is safe and attended at all times, especially if there's props around they may decide to wake up and play with. Try not to get carried away with the backdrop as you may end up having a backdrop that is stronger than your tiny subject.
Above all, be patient and just keep shooting, the perfect photograph is there, you just have to wait for it!