If your film is set in the 1940s (or any period), does the audience believe it? Obviously the costumes are part of the overall production design, so don't skimp on making it look real. Do online research for photographs from the period. Much of the clothing up until the 1950s, were made of heavier cloth and certainly not the stretch fabrics of today. 1940s military uniforms were akin to wearing sackcloth. Trips to Salvation Army or other thrift stores yield absolute bargains and you will often find 1960s era coats in a heavier fabric.
Costumes on a low budget
Accurate WW2 costumes are hard to get and the reproductions, whilst accurate, are fairly expensive for a complete ensemble ($350-$500 for a German outfit). We team up with local military reenactors who are very well versed in all the minute detail needed for WW2 era uniforms and kit. Many reenactors will rent their gear and we used two on our Scheldt feature film to act as coordinators for German and Canadian background cast. We also rented gear from IMS in Vancouver who ship military costumes all over North America.
Do actors know how to use their gear?
Make sure you train your cast how a soldier should use and handle their gear including how to adjust web gear, helmets. Its easy to see the films where an actor is only wearing the costume vs being a soldier. Another key point, much like actors really reading a book or drinking real coffee - equipment with foam in the pouches looks just like that. Military web gear is heavy when fully loaded (35lbs and up!) so fill the packs and pouches with weight.
They look too clean!
This is a common complaint. Many films show soldiers who look like they just came from a training depot rather than having spent 4 weeks in a trench. Have your cast wear the costume all day. Don't hang it up after each day - just leave it in a crumpled pile or in a garbage bag. Wrinkled clothing with sagging pockets is the most authentic. Don't overdo the mud and dirt either! Soldiers are particular about trying NOT to get dirty - they have to live in muck and filth so they try their best to avoid it.
Military costumes, War films, period costumes