I use the different folders method which I keep on my desktop. However, as an example of what works for me, I've named folder #1 called CanStockPhoto, FP, StockXpert, LO. The second folder is named CanStockPhoto, FP, StockXpert. The third one CanStockPhoto, FP. The fourth one CanStockPhoto. I put all the new images in folder number one and upload to LuckyOliver (the last initials in the name of the folder). Then I move everything to folder #2. Repeat the process by uploading everything to StockXpert (the last initials in the name of folder #2). Move contents to folder #3 and so on. I put CanStock at the end because of the four listed, they are the only one with upload limits. Therefore, the pictures can continue to sit in the last folder for as long as it takes but it isn't holding up these photos being uploaded elsewhere.
I do upload to several others and have different sets of folders for them following the same guidelines as above. Since IStock has such a small upload limit, this is obviously my last folder in the other grouping.
I also keep the folders currently being uploaded open with the pictures highlighted until the upload is done. Then I immediately move the highlighted pictures to the next folder and close the folder they were removed from.
Sounds confusing but it works for me. Hope this helps.