On co-sleeping with babies...

Safe Bed

• large enough to accommodate mum, dad and baby
• king size mattress placed on the floor away from wall would be ideal
• flat and firm mattress
  

Other Bedding Concerns

• use a fitted bed sheet
  
• avoid extra pillows, comforters or blankets
  
• no bedding items for new born (they should be dressed warmly)

Where Baby Sleeps

• baby should be put between mum and the edge of bed (if on floor), wall or guardrails
• if mum is a very sound sleeper, a crib next to the bed is a much better option.

No Smoking, Drinking and Drugs
Parents who are sick and on medication should also avoid co-sleeping (risk spreading the virus to baby; medicines consumed may put her into a deep slumber, making it difficult for her to wake up in the night if the baby cries.)

Size and Weight Issues
Bed sharing definitely becomes more dangerous for the baby if one or both of her parents are large and overweight. If the baby rolls towards mum or if mum’s weight leaves a big impression in the mattress when both are on the mattress, the chances of the baby being crushed and smothered are very high.

Sources:
http://infant-toddler-health.suite101.com/article.cfm/bed_sharing_safety_tips