Firstly the most obvious is to continue breastfeeding when you return to work. Even if you don't want to continue this longterm - it can be gradually phased out as your child adapts to childcare (although as long as you continue to feed, it reduces illness length and severity - so can be a godsend to working mums in terms of them having to be absent to care for an unwell child) If your baby is younger than six months, trying to continue exclusively with human milk until the gut closes - can have an extremely significant impact.
Depending upon the age of your child and how long you are apart - there are lots of options. You can choose to pump and send expressed milk, use a combination or just feed around work when with baby; this means you can still have all the flexibility and closeness with baby when at home. Many mums comment it is a lovely way to reconnect after separation, and many babies make it obvious they agree.
Most if not all infants have "settling sessions" at their new setting before being left. Use this time to get down to floor level (easy in an environment with children) and handle what your baby does and will touch. If toddling age you're also at perfect height to directly come in contact with peers too. If you have a baby touch cots, highchairs - anything you can discreetly make contact with (without making yourself look a bit of a mentalist with a wiping obsession ;)) Any exposure you get to nasties will trigger your body to make appropriate antibodies, which can then be passed back in breastmilk.
Continue this as much as you can when collecting and dropping off, particularly at any mention of a bug. If the environment allows even plonk yourself down and feed there before you leave. If your partner is doing the collecting get them touching things too, then get close to them!
If you are sending milk with baby, when they first start use as much milk as you can that was expressed after contact with the new surroundings. Again if feasible for the initial period. try and use milk that hasn't been frozen as it's higher in immunological factors. Once baby is settled if they pick up a bug, reverting to freshly expressed if you normally freeze can help.
Initially when mums return to work, baby may feed a lot more frequently when reunited This may be due to a reduced intake in their daycare setting or because they are craving closeness and reassurance, but the bonus of this is it not only maximises opportunity for baby to pass to you, any germs he has been exposed to, but also allows lots of opportunity for you to pass the relevant antibodies back.
Lastly always ensure your care provider knows how to thaw, heat and store your breastmilk. If they treat it as formula you may find a lot of unnecessary waste, and demands for milk far beyond levels of consumption. Freeze in small amounts so there is no need to defrost a large amount and discard if not required.