If you wish to play in a campaign, it is a good idea to fill out a unit roster card for each squad and vehicle in your army (unit roster cards like the one below can be photocopied from the examples included in the Appendix). This card then becomes a record of the unit over your following battles. As units fight they will lose or gain experience and any new skills or abilities they gain or penalties they suffer from are noted on the card. If you make any changes to the composition of the unit you alter the card to show this, and make up new roster cards if you add any more units to your army.
When you come to choosing your army for a battle, rather than picking it from scratch from the army list each time, you choose which units to use from your file of rosters. So, for example, if you are fighting a battle that allows you to field three Fast Attack choices, rather than picking fresh units from the army list, look through the roster cards you have filled in for your Fast Attack units and decide which of these you want to use. Keep choosing units from your file of rosters until you have the army you want. Think of your unit roster cards as building bricks which you can combine together in different ways to achieve a varied army choice.
The main point is that individual units become regular components of your army, rather than being one-off choices for a specific battle. Some of them will be tried and tested veterans, whilst others will have yet to prove their mettle. In this way, over a number of battles your army will start to develop its own character with its particular strengths and weaknesses, which will differentiate your army from others, even of the same race.