Give yourself time – you may live to regret making a hasty decision so it's worth wearing comfortable clothes, taking off your coat and shoes off and spending at least 10 minutes really relaxing into your potential new mattress. Try your normal sleeping positions and lie on your sides and back as well.
The key in finding the right mattress is to focus on two factors - comfort and support. Manufacturers usually sell firm, medium and soft options of mattress. You might want to try all three to ensure that you find the one that's right for you. Lie on your back on the mattress and put your hand under the small of your back. If there is a gap between the mattress and your back, then the mattress is too firm which may lead to discomfort at the pressure points such as the hips and shoulders. If your back is rounding into the mattress (as if you were lying in a hammock) then it is too soft which will offer little support and possibly result in back pain and postural changes. If your hand stays in contact with the bed and your lower back at the same time then you're nearer to finding the right one.
Crucially, the mattress should mould to the shape of your body and feel supportive. When you are lying on your side, your spine should remain horizontal all the way from your neck to your lumbar spine and not dipping down towards the floor other than a little at your waist as it's naturally lower than your hip height (this is where it is important to make sure that you also find the right pillow so that this position can be achieved). You should also be able to turn easily.
Remember that you may need different mattresses if you and your partner are different sizes and weights. Look for double mattresses where two single mattresses of different firmness are zipped together or there is a combination of firmness on each side of the same mattress. The heavier the person is the deeper the mattress needs to be – and it's best to go for one that has firmer springs.
If you are buying a new mattress, also consider buying a new base as a good mattress on a weak, old base will not effectively support your back or be comfortable and also invalidates the guarantee of the mattress manufacturer. Go for a good quality full sprung edge base.
Are you suffering from back pain? If so, you may be tempted to go for an 'orthopaedic' mattress. This usually simply means that it is the firmest mattress in that particular range, which may not be the best option as recommended by a back specialist. Again, the most important factors are comfort and support - the mattress should fulfil the criteria mentioned above.