A lot of that depends on the age of all the children and the custody arrangements of the step-children.
Simple things like being as consistent as possible in all things such as rewards, chores or punishments. If you'd punish your own child for something, the step-children should be treated the same if committing the infraction. If you reward your own child for something or the step-children for something than the reward should be accepted as something they can all earn. The biggest one to keep all children feeling like one isn't being treated any different than the others, is to divide chores up evenly...even if the step-children are not there 24/7, they should still have some responsibility while they are there. They should feel like part of the family, not like a visitor. Not to say that you should save all chores for the weekends that they are there so they can participate, but more of dinner's ready, child A - set the table and Child B - clear the table & Child C - it's your turn to help with the dishes...I think you get the point.
Have family meetings where everyone gets at least a couple of minutes to have their own say...whether they talk about things they want to do or problems they are encountering, it helps them feel like they are being heard.
Realize too that no matter how hard you try to be equal handed or treat them all the same, at some point all children feel like they are treated differently...some of that is age related (1 year olds need more attention than 13 year olds...and 15 year olds get more privileges than 13 year olds) and sometimes kids will compare everything possible and if you messed up even once, they can make a mental note of it to throw at you when you least expect it.
Try to spend a little bit of time alone with each one...even if its 5 minutes at bedtime reading a book, singing a song, praying or for older ones asking about their day. I find that hard to do sometimes.